PB February-March 2026
February-March 2026 41
By Mark Tottenham
Expert Evidence on Trial
Lucy Letby case suggests need for:
legal aid for equal expert evidence
for defence; code for experts; and
facilitation of joint reports
Letby either is a serial
killer or dedicated nures
on duty when sick
children died of natural
causes
L
ucy Letby, if the prosecution case is
to be believed, is a serial killer with
a grotesque appetite for murdering
premature babies under her care,
before attempting to comfort their
distraught parents. This was accepted -
beyond a reasonable doubt - by the trial jury
(and by another jury, in a retrial on one of
the counts). The Court of Appeal, in a careful
judgment, considered the conviction safe.
If the defence — including her new legal
team, brought in after her unsuccessful appeal
— are to be believed, she was a dedicated
nurse who frequently worked overtime in a
badly run neonatal unit, and who happened to
be on duty when a number of very sick children
died of natural causes. This is also the view of
an international panel of medical experts with
glittering credentials.
The deaths occurred between 2015 and
2016, when Letby was in her mid-20s. Her
trial concluded in August 2023, when she
was convicted of seven counts of murder
and seven counts of attempted murder. She
received a ‘whole life’ sentence, meaning she
is expected to die in prison.
The only person who knows for sure whether
Letby deliberately caused these deaths is
Letby herself. No amount of retrospective
scientific analysis is likely to establish her guilt
or innocence with absolute certainty.
That said, the case raises serious questions
about the use of expert evidence in criminal
trials. Irish courts rely on the same adversarial
model of expert evidence, with similar
pressures on judges, juries and legal aid, so
the questions are of direct relevance here.
The prosecution expert who
volunteered his services
The prosecution’s central expert witness was
Dr Dewi Evans, a retired paediatrician from
Wales.
Although he was not a neonatal expert,
he had been the director of a large neonatal
unit in Wales. Any issues concerning his
qualifications were dismissed by the trial
judge, and the Court of Appeal agreed. Given
the gravity of the allegations — serial murder
by a healthcare professional — it might have
been thought that a more specialised expert
could be recruited, but there were no ‘red
flags’ concerning his qualifications to testify.
Evans’s report was said to have been ‘peer
reviewed’ by two neonatal specialists. One, Dr
Martin Platt, supported Evans’s conclusions
but died before the trial. The other, Dr Sandie
Bohin, had retired from neonatal practice and
was working in Guernsey, but also endorsed
his findings. How extensive or rigorous this
peer review was remains unclear, but it did not
worry the Court of Appeal.
The trial itself lasted almost a year. The
Crown Prosecution Service has estimated
prosecution costs alone at over £2.8 million.
The most disquieting issue concerning Dr
Evans emerged during the trial, and concerned
his suitability to act as an expert witness,
rather than his medical qualifications. It
transpired that he had been the subject of
severe judicial criticism in another case. Mr
Justice Peter Jackson stated:
“Dr Evans makes no eort to provide
a balanced opinion… The report has the
hallmarks of an exercise in ‘working out an
explanation’… It ends with tendentious and
partisan expressions of opinion that are
outside Dr Evans’ professional competence
and have no place in a reputable expert report”.
These comments were drawn to the attention
of the trial judge in Letby’s case. He declined to
exclude Evans’s evidence, but allowed the jury
to be informed of this criticism.
The same issues were raised on appeal, but
the Court of Appeal rejected them.
(Since then, incidentally, the High Court of
England and Wales (in SC Commercial Bank
Privatbank v Kolomoisky [2025] has ruled that
experts are themselves under a duty to inform
the court if they have been the subject of
previous judicial criticism.)
Given the stakes in the case, it is dicult
not to feel that Evans may not have been the
ideal person to lead the expert evidence for the
prosecution.
The defence expert who never
testified
Letby’s defence team instructed Dr Michael
Hall, a retired consultant neonatologist. He
was never called to give evidence.
Hall has since told the media that he does
not know why this decision was taken. It can
only be speculated that there was something in
LAW
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42 February-March 2026
February-March 2026 PB
Defence expert Michael
Hall noted the paper
concerned the injection of
oxygen rather than air —
despite its title. Strangely
he did not give evidence
his report the defence did not want ventilated
before the jury.
Lawyers are often accused of “expert
shopping” when they abandon an opinion they
do not like. But in a case of this magnitude, it
is at least arguable that the legal-aid system
should have facilitated more in-depth expert
consideration.
The prosecution theories
Dr Evans identified air embolism as the likely
cause of death in several cases, concluding
that Letby had deliberately injected air into
the babies’ veins. In others, he alleged insulin
poisoning or force-feeding with milk. None of
these causes were identified in post-mortems
at the time. They only emerged after the spike
in deaths was retrospectively examined for
signs of deliberate harm.
Because it is ethically and practically
impossible to conduct the necessary
experiment, there is little published material
on air embolism in newborn infants. Evans
relied heavily on a 1989 academic paper,
Pulmonary Vascular Air Embolism in the
Newborn, by Lee and Tanswell.
One of defence expert Michael Hall’s
criticisms was that the paper concerned the
injection of oxygen rather than air — despite
its title. The jury never heard this evidence.
After Letby’s conviction, her lawyers sought
to introduce fresh evidence from one of the
paper’s authors, Dr Shoo Lee, a Canadian
neonatologist. Dr Lee stated that his work had
been misinterpreted, and that alleged skin
discolouration in some of the children as relied
upon by Evans had been misunderstood.
The Court of Appeal refused to admit this
new evidence, partly because the defence had
not adequately explained why Dr Lee had not
been called at the trial itself.
Dr Lee then took an unusual step. In February
2025, he appeared at a press conference
organised by Letby’s new legal team,
announcing that he had convened an unpaid
international panel of experts from a range of
disciplines to review the medical evidence in
relation to all of the deaths. Their conclusion
was stark: no murders had occurred, and all
deaths were attributable either to natural
causes or to poor medical care.
An application based on this report was
made at the time to the UK’s Criminal Cases
Review Commission (CCRC), whose role is
to refer cases to the Court of Appeal if there
is evidence of a miscarriage of justice. At the
time of writing, ten months later, no decision
has been made.
It may be that, taken with the other
circumstantial evidence, the prosecution
theories are suciently corroborated that the
CCRC will not feel there is any necessity to refer
the case back to the Court of Appeal. But it is
dicult to ignore a panel of world class experts
who feel that the matter has been poorly
handled.
Trial by media
One of the most striking features of the Letby
case has been the extent to which the expert
witnesses and lawyers have aired their views
publicly, in everything from learned journals to
documentaries.
Most prominent of all has been Evans
himself, who has repeatedly asserted Letby’s
guilt in broadcast interviews. Expert witnesses
are required to be independent, owing their
duty to the court rather than to the side that
calls them. They are meant to assist the jury,
not replace it.
Letby’s current barrister, Mark McDonald,
has also spoken publicly about the case,
expressing equally firm views on her innocence.
Yet when asked (by me) at the Bond Solon
Expert Witness Conference in November about
expert witnesses engaging with the media,
he said they should never speak to the press.
Doing so, he said, risks undermining their
independence and breaching professional
privilege.
Explaining complex science to
juries
Cases involving complex medical or scientific
evidence pose particular dangers. This was
famously exposed in the Birmingham Six case,
where the trial judge admitted to the jury he
did not understand the forensic science, and
instead suggested that they should defer to
the prosecution expert’s seniority. This was
the only evidence in the case apart from the
defendants’ own forced admissions, and it
took 16 years to establish that the science he
presented was wrong.
Other jurisdictions have experimented
with solutions, such as pre-trial educational
sessions for judges and juries, or simplified
explanatory materials. An interesting example
was the preparation of comic books on DNA by
Dundee University. In civil cases, it is common
for experts on either side to meet and prepare
joint expert reports, clearly setting out areas of
agreement and disagreement.
There is no obvious reason why such joint
expert reports could not be prepared for
criminal trials, in cases where the defence
intends to call its own expert witness on a
contentious matter.
Suggestions for future trials
1. Given the enormous cost of complex
criminal trials, investing early in the most
appropriate and thorough expert evidence is
likely to save money — and reduce potential
injustice — in the long run.
2. The legal aid system should ensure that
the defence has access to equally robust
expertise, particularly in technically complex
cases.
3. Judges should be supported in engaging
with expert evidence before it is presented
to juries. Where feasible, joint expert reports
could help clarify what is genuinely in
dispute.
4. A formal code of conduct for expert
witnesses should include clear restrictions
on public commentary until the risk of retrial
has eectively passed.
Mark Tottenham is a barrister and expert
witness advisor. He will be speaking on ‘Expert
Witnesses and Miscarriages of Justice’ at the
Kilkenny Law Festival, 6-8 March 2026
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