“My evidence might have changed the trial against Lucy Letby,” says an expert

An expert witness for the defence in the trial of Lucy Letby has said he was “disturbed” that he was not called to give evidence and believes the outcome may have been different if he had done so.

“I wrestled with that for a while,” said Dr. Michael Hall, a neonatologist. “I felt the jury had not heard the whole truth.”

Letby was convicted of murdering seven babies while on duty at the Countess of Chester Hospital. A jury also found that she had attempted to murder six more. In a retrial, she was found guilty of the seventh attempted murder after the original jury could not reach a verdict on one count. She is serving a life sentence prison sentence.

The original trial, which took place last year, was one of the longest murder trials in English legal history. It followed a police investigation that examined over half a million medical documents and involved more than 2,000 people. Two appeals were rejected.

Although two juries found her guilty, some experts have since expressed doubts about the strength of the evidence. Hall was hired by the defense as an expert witness, but was not called at the trial.

He said that had he been on the witness stand, he would have questioned some of the prosecution's claims. “I would have given different answers to the prosecution's medical experts and interpreted some cases differently,” he said. “That is not to say that I know all the answers or that I know Lucy Letby is innocent. There were certainly some events that were difficult to explain.”

“But the evidence was, in my view, contestable.”

In particular, he disagrees with the claims about the health of some babies before their deaths. “I believe that the prosecution's medical experts have exaggerated when they say that some babies were 'completely stable' before their collapse.”

He also argued that the evidence for the claim that some babies died from air injections was relatively weak, relying heavily on a research paper that actually involved oxygen injections, even though air is mostly nitrogen. “The important thing about this is that oxygen and nitrogen behave differently in the body. Nitrogen doesn't cross membranes very well. It's an inert gas.”

• How strong is the evidence against Lucy Letby?

Hall, whose advice influenced the defense's strategy even though he did not provide evidence himself, said he did not know why he was not called. The lawyers who retained him cannot comment because the decision is subject to attorney-client privilege.

Mark Solon, a lawyer who runs a training company that teaches expert witnesses how to work in the legal system, said there could be several reasons. One plausible explanation is that the defense felt he had not done well under cross-examination – perhaps because he had not clearly affirmed Letby's innocence. Another is that they felt they had already laid out their case.

“Maybe they thought, 'Well, we've disproved the charges enough that we don't have to turn anyone against them anymore,'” Solon said.

“You don't want to overdo it with eggs. When you think you've got it, you don't want to put more eggs in. With evidence, sometimes less is more. There's a risk of confusing the jury. So it's a tricky business.”

Hall said whatever the reason, he felt it was a mistake not to have been called.

“The majority of the evidence was circumstantial,” he said. “Nobody saw her do anything and there was little forensic evidence to support the allegations.”

“As I understand it, the judge will have told the jury that they have to be sure to convict or find them guilty. It's not a question of 'virtually sure' or 'almost sure'. It's a question of 'sure.'”

He said he had explained that in his view there were still doubts. “It seems to me that the failure to call experts could have had an important influence on the jury's outcome.”