How often do you see an expert witness themself become part of the reason a criminal conviction is overturned?

That is, in effect, what happened in The King v ZX [2025] NICA 76, a decision of the Court of Appeal in Northern Ireland concerning convictions for murder and attempted murder.

Psychiatric evidence was central to the trial. The defendant’s mental state was directly relevant to the issues the jury had to decide, and both the prosecution and defence relied on psychiatric experts.

What makes the case particularly striking is not simply that the Court of Appeal disagreed with an expert’s opinion. Courts do that all the time. The problem went much deeper. The court concluded that the prosecution psychiatrist’s evidence had evolved in such a way that it could no longer be regarded as the independent product of an expert uninfluenced by the demands of the litigation.

The expert had, in the court’s words, effectively assumed an impermissible role in promoting the prosecution case.

That is an unusually serious finding against any expert witness.

An opinion which developed during the trial

Before trial, the prosecution psychiatrist had assessed the defendant as suffering from depression, although not to the degree suggested by the defence expert. During his oral evidence, however, he introduced a much more significant proposition: that the most likely explanation for what had happened was “spousal revenge”.

That theory had not appeared in his reports or his contemporaneous notes. It emerged in evidence after the expert had been provided with research material addressing that subject by prosecution counsel.

There were other examples of important matters appearing for the first time at trial. The expert placed weight on the condition of the defendant’s home as an indicator against severe depression, although that had not featured in his reports or notes. He also relied on the severity of her self-inflicted injuries in a way which had not previously been identified.

Experts are, of course, allowed to change their opinions. New information may emerge or further consideration may lead to a different conclusion. But a material change of view should be identified and communicated. A party should not discover an important new expert opinion for the first time when the expert is giving evidence.

That principle was central to the Court of Appeal’s reasoning.

A quotation which changed meaning

Perhaps the most uncomfortable factual detail concerned a quotation attributed to the defendant.

The expert’s report recorded her as saying:

“he destroyed my life, so I destroyed his”

When the expert’s original handwritten notes were eventually disclosed during the trial, however, they recorded only:

“he destroyed my life”

There was also a discrepancy about when the statement was said to have been made.

The difference is obvious. Adding the second half of the sentence changes a statement about what had happened to the defendant into something which appears retaliatory. That was particularly significant given the later emergence of the “spousal revenge” theory.

The court recorded that, when challenged in cross-examination, the expert at one stage agreed that he had lied about what had been said, although he later resiled from that position.

Whatever the explanation, it illustrates why an expert’s original notes can matter just as much as the final report.

Material which had not been considered

The problems were not confined to the development of new theories.

The expert accepted that he had not considered police body-worn footage, had not asked to see it and had not regarded it as part of his assessment. He had also failed accurately to record when, and to what extent, the defendant had received antidepressant medication. When these omissions were put to him, he maintained that they did not alter his opinion.

That poses a familiar problem for experts. A conclusion may ultimately remain the same after additional material is considered, but that is different from never having considered the material in the first place.

An expert needs to be able to explain not only the conclusion reached but the evidential basis on which it was reached, including material which points in the other direction.

The relationship between the expert and the legal team

The Court of Appeal was also highly critical of the way in which the prosecution expert had been handled.

Before giving evidence, the expert had a telephone conversation with prosecution counsel lasting between approximately 45 minutes and an hour. Neither participant made a note or record of the conversation. A further meeting took place on the day the expert began giving evidence.

The concern was not simply that discussions had taken place. Conferences between lawyers and experts are normal and often essential. The difficulty was what happened afterwards.

The court noted that matters raised during the consultation subsequently became prominent in the expert’s evidence and increasingly supportive of the developing prosecution case. It said this gave rise to an appearance of coaching or of the expert having been consciously or subconsciously influenced.

Importantly, the court did not make a finding of deliberate coaching, but it did find that the prosecution’s conduct had fallen below recognised standards of good practice.

There is a useful distinction here. Lawyers are entitled to test an expert’s opinion, challenge assumptions, ask questions and introduce new material. What they cannot properly do is shape the expert’s evidence into the case they want to present.

The expert has the corresponding responsibility to resist that process.

No written letter of instruction

Remarkably, the prosecution psychiatrist had no written letter of instruction.

The Public Prosecution Service explained that he had been given a brief verbal explanation of the case, but nothing had been put in writing.

The Court of Appeal’s response was blunt. It described that as “patently not good enough”, said that the practice lacked transparency and stated that in future experts in criminal cases should have a written letter of instruction.

For experts used to civil litigation, that may sound like an obvious requirement. But the reason for it goes beyond administration.

Written instructions establish what the expert was originally asked to consider. They help define the issues, provide a record of the material supplied and make it possible later to see how and why an opinion developed.

Without that starting point, it becomes much harder to distinguish independent analysis from a case theory which may have developed through conversations with the instructing team.

The court’s conclusion on independence

The Court of Appeal returned to well-established principles governing expert witnesses.

Expert evidence should be the independent product of the expert. An expert should provide objective and unbiased assistance to the court, should identify the facts and assumptions supporting an opinion, should not omit material facts which detract from it and should communicate any material change of view promptly.

Applying those principles, the court concluded that the prosecution psychiatrist had departed significantly from his role as an independent expert and had effectively assumed an impermissible role promoting the prosecution case.

The consequences were serious.

The court said that without the perseverance of defence counsel in obtaining the expert’s notes and examining his dealings with prosecution counsel, there was a real possibility that the deficiencies and partiality in the evidence would never have been exposed. It said that this could have led to a miscarriage of justice.

The ground of appeal relating to the expert evidence succeeded.

The responsibility did not stop with the expert

It would nevertheless be wrong to treat ZX simply as a case about an expert who lost his independence.

The Court of Appeal also found serious problems in the conduct of the prosecution and in the way the trial judge dealt with the difficulties once they had emerged.

The prosecution’s handling of the expert was criticised, including the informal nature of the instructions, delayed disclosure and the interaction between counsel and the expert. The ground of appeal relating to prosecutorial misconduct also succeeded.

The court also found that the trial judge had failed to give sufficient consideration to applications to discharge the jury. It concluded that the jury could easily have been misled by the expert evidence and that directions could not have cured the resulting prejudice.

Both convictions were quashed.

The court’s overall conclusion was that the serious errors began with the prosecution’s misconduct and were compounded by the expert witness and the judge.

Could a new expert start again?

There is one final aspect of the case which is particularly interesting for anyone involved in expert evidence.

Once the convictions had been quashed, the court had to consider whether a retrial could take place. One difficulty was that the “spousal revenge” theory had by then become embedded in the case papers.

The Court of Appeal was concerned that any new expert might themselves be influenced by that history simply by reading the existing material. It described the risk of the replacement expert becoming “contaminated”.

When a retrial was eventually ordered, the court directed that the new expert’s letter of instruction should, so far as possible, be prepared on a “blank page basis”.

That is perhaps one of the most thought-provoking parts of the judgment.

Expert independence is not only about resisting pressure from the party instructing you. It also means being alert to the way in which the material provided to you can frame the problem before you have even begun your own analysis.

Independence can be lost gradually

Most experts know that their duty is to the court rather than to the person paying their fees. It is one of the first principles of expert witness work.

What ZX demonstrates rather starkly is how independence can be eroded without there necessarily being one obvious moment when the line is crossed.

A discussion takes place. A new theory is raised. A piece of research is introduced. Something which seemed insignificant in the report becomes central in oral evidence. Material which does not fit the emerging theory receives less attention. Gradually, the expert stops simply assisting the court and begins to move with the case being advanced by one side.

That is why the disciplines around expert evidence matter: clear written instructions, complete records, contemporaneous notes, proper disclosure, transparency about changes of opinion and a willingness to consider evidence which does not support the preferred conclusion.

None of those things is glamorous. They are, however, part of what keeps an expert independent.

In ZX, their absence contributed to something very unusual indeed: the expert’s own conduct became part of the reason two criminal convictions could not safely stand.

Case: The King v ZX [2025] NICA 76, Court of Appeal in Northern Ireland.

Read the judgment

The Criminal Procedure Rules Committee have issued a new consolidated set of the Criminal Procedure Rules which came into force on 6^th October 2025.

The Criminal Procedure Rules 2025 replaces the Criminal Procedure Rules 2020 and incorporates the 9 sets of amendments made since the 2020 rules were published. The Committee has also published a guide to the new Rules. criminal-procedure-rules-2025.pdf

In addition, the Criminal Procedure Rules Committee have just published the. Fourth Amendment to the Criminal Practice Directions 2023 – November 2025 – Courts and Tribunals Judiciary. The changes, some of which will impact on experts in Chapter 7, come into force on 18^th November 2025.

The amendments:

1. require the disclosure in an expert report of any authoritative advice or warning relevant to the content of the report;
2. require the disclosure in an expert report of any past adverse finding or disciplinary proceeding, even if since resolved; and
3. ensure the compatibility of the declarations of compliance required by (i) the Criminal Practice Directions, and (ii) the Forensic Science Regulator’s statutory code of practice.

Updated Expert Declaration
In addition to the changes outlined above there are also changes that need to be made to the Expert’s Declaration, full details are on the Academy website.

 

Don’t forget to update any templates you have that are effected by the changes and to satisfy yourself that any report issued after 18^th November 2025 complies with the new requirements.

Being instructed as an expert witness can be an interesting and fulfilling aspect of professional life, but it’s not without its challenges. One of the most common (and under-discussed) issues experts face is how to manage their instructing solicitors.

It’s easy to assume that once you’re engaged, expectations are fixed and your role is simply to deliver. But in reality, managing your instructing solicitor is a key part of ensuring that what you deliver is useful, compliant, and capable of standing up in court.

Academy Council Member Helen Gregory of Opus Forensic Accounting has helpfully provided her five top tips for how expert witnesses can work most effectively with their instructing solicitors.

1. Be realistic about deadlines — and say no if needed

There’s a temptation to take on every instruction that comes your way, particularly if you’re self-employed and don’t know where the next one is coming from. But accepting unrealistic deadlines can cause problems for all involved. Rushed reports are more likely to contain errors, lack clarity, or fail to meet procedural requirements. And in the worst-case scenario, they can be thrown out by the court entirely.

It’s better to decline a job than to submit something half-baked. Be clear at the outset about what’s feasible and remember to factor in time not just for writing the report, but for reviewing documents, requesting further information, meetings with the solicitor and client, responding to queries, and navigating any surprises.

2. Build in contingency: Something will crop up

In theory, you receive a neat, complete bundle of documents and a clear brief. In practice, things are rarely that tidy. Documents arrive late or in multiple batches, some are missing or out of order, and the questions you need to address will take time to answer.

You need to allow for contingency: time to request clarifications, review additional information, and think critically about what you’re being asked to do. The more flexibility you build in, the more likely you are to produce a report that is robust, complete, and helpful to the court.

3. Understand the limits of your visibility

One key challenge is that, as an expert, you often operate in isolation from the wider case. You may see only a snapshot of the full picture. It’s important to remain focused on your instructed area and resist the temptation to speculate beyond it.

At the same time, remember that lack of context doesn’t remove your duty of care. If documents appear incomplete, if your opinion feels influenced, or if something doesn’t sit right, you must raise it. You are reporting to the court, not acting as an advocate for either party.

4. Deliver bad news where it’s warranted

It’s not always easy to say your client doesn’t have a case. But as an expert witness, you have a duty to give your honest, professional opinion, even if that opinion isn’t what the instructing solicitor wants to hear.

Be aware that some may (consciously or otherwise) attempt to steer your opinion. Stay alert to missing evidence, biased questions, or subtle attempts to frame a narrative. Your integrity and credibility depend on your ability to remain independent and evidence-based.

5. Know the rules and review your report thoroughly

Expert reports must comply with the relevant Civil, Criminal or Family Procedure Rules. That includes formatting, content, and declarations. A technical report that omits the correct statement of truth or includes the wrong version risks being ruled inadmissible.

It’s also essential to re-read what you’ve written. That may sound obvious, but when deadlines loom and multiple drafts are flying around, it’s easy to miss a critical detail. The best option is to ask someone else unconnected to the case to read and cast it, but that may not always be possible so do make time to review your own work with a clear head.  You may be cross-examined on your report, so make sure you’re comfortable defending it.

Final thoughts

Whatever your discipline, acting as an expert means more than just knowing your subject. It requires planning, time management, self-awareness, and the confidence to push back when needed.

Clear communication with instructing solicitors, realistic boundaries, and careful preparation are vital. If in doubt, The Academy of Experts offers training, guidance and support to help navigate the more complex aspects of expert witness work.

Helen is a Partner at Opus Forensic Accounting, part of the Opus Business Advisory Group, and brings over 25 years’ experience in civil and criminal investigations, commercial disputes, and matrimonial matters. A trusted expert witness and active figure in the sector, including roles with The Academy of Experts and the South West Fraud Forum, she recently joined Opus, a national advisory firm with 14 offices, 35 Partners, and specialist divisions spanning restructuring, strategic advisory, forensic accounting, and equity.

 

Two recent articles in the Guardian cited the appeal court’s over-turning of a doctor’s conviction and an assortment of criticisms of experts made over recent years in highly critical articles, calling for compulsory training and registration of experts. Our Chair, William Hooper responded on our behalf as follows;

Recent articles have discussed concerns about expert evidence in the Courts and argued for greater regulation.

An expert witness is someone with specialist knowledge which the Court considers useful to itself in resolving a case.  The expert’s task is to explain the significance of technical material to the Court so that the judge (in a civil case) or jury (in a criminal case) can understand it and include it as part of their considerations. The decision-makers weight evidence, including that from experts, as they see fit.

The expert witness does not decide the case in either the civil or criminal courts.

Technical evidence can be complex.  It often happens that expert witnesses appointed by the two sides agree on some points of interpretation and disagree on others.  Expert witnesses may be subjected to robust cross-examination to detect error, incompetence or the hired gun.

Your writer calls for greater regulation but appropriate regulation has been introduced.  Instructing solicitors are required, when instructing experts, to inform them of their duties under the Civil (or Criminal or Family) Procedure Rules and the Practice Directions.  These cover the minimum legal obligations for expert witnesses.

The Academy of Experts, since 1987 a professional and accrediting body for expert witnesses, agrees that training is appropriate to ensure that those appointed fully understand their obligations and how those apply in a variety of potentially difficult situations.  Such training has been available for many years from The Academy of Experts and other sources.

Rather than the imposition of additional regulation, which may result in the exclusion of experts with the best experience and expertise to assist the Court, it is to be hoped that the CPS and instructing lawyers will carry out appropriate due diligence and choose to instruct properly qualified and trained expert witnesses.

William Hooper

Chair, The Academy of Experts

3 Gray’s Inn Square, London WC1R 5AH

As you may be aware the validity of computer evidence has been in the news recently because of the public enquiry into the Post Office and its Horizon system, which formed the basis of prosecutions  of sub-postmasters.

Earlier this year the Ministry of Justice issued a call for input on the use of evidence generated by software in criminal proceedings.The MoJ wished to understand how the current presumption concerning the admissibility of computer evidence is working in practice, and whether it is fit for purpose in the modern world. As the leading professional and accrediting body for expert witnesses in the UK and world-wide, The Academy of Experts convened a working group that included experts in IT and computer systems and a range of other areas of expertise who use computer-generated evidence in criminal matters. The group debated the questions posed by MoJ and developed its view to form a response.

It is the view of The Academy that the Presumption should remain, but it is vital to recognise its limitations and to welcome challenges to it. Issues of disclosure are central to addressing this situation. The reasoning and recommendations we made are set out in full in our response to the consultation.

We are grateful to everyone who contributed to the working group;  Mark Ballamy, Richard Emery, Mark Holdsworth, William Hooper, David Tonks, Gill Hunt, PeterMcArthur, Nigel Young, Jawad Hassan Zadeh