Gareth Jenkins was the architect of the Horizon IT system at the heart of the Post Office scandal, responsible not just for building it, but for defending it in court. 

As an expert witness, he was deployed by the Post Office to rebut accusations that it was the computers, not the sub-postmasters, that were making money disappear from branch accounts.

As a consequence, he is under police investigation for suspected perjury.

In a number of prosecutions he denied that there were “bugs, errors and defects” in the wider Horizon system.

That makes him a crucial witness to the public inquiry, so important that he’s tabled to give evidence for four days, one more even than the former Post Office chief executive Paula Vennells.

Defensive opening

Unlike many of the former Post Office and Fujitsu employees who have given evidence so far, he chose not to begin with an apology to those whose lives and livelihoods have been ruined.

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Instead, he chose to defend Horizon. Asked about the High Court ruling that paved the way for the clearing of hundreds of sub-postmasters, he rejected the judge’s conclusion that the system was “not at all robust” and insisted it had worked well.

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“There were clearly problems during the pilots, and there were clearly individual problems that affected individual branches… but in general, then I felt that the systems were working well.”

Mr Jenkins did apologise in his witness statements to former sub-postmasters Seema Misra and Noel Thomas, who were both wrongly convicted and imprisoned.

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Grey-bearded with salt-and-pepper hair, giving evidence in a white shirt and charcoal jacket, Mr Jenkins may know more about Horizon than anyone else.

That knowledge led the Post Office to increasingly rely on him for advice as complaints and concerns about the tide of prosecutions based on Horizon evidence began to pile up.

When examining a case he said and treated each one like his “day job”, looking at the specifics of the particular branch accounts and examining if there was any evidence of a bug.

Not aware of system problems

He told the inquiry that initially he was not aware of problems in the wider system, because it was not his job to deal with them, and he did not think to ask, even when giving witness statements and evidence in court.

“What I was aware of was the fact that bugs that actually impacted the accounts were very rare,” he said.

“There was good monitoring in place to detect them, and they got fixed shortly afterwards. So in terms of what was actually there in the live system at any one time, it was very rare for there to be bugs there that would cause problems.

“Obviously, with hindsight, I realise that maybe I should have been doing more research.”

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Wider issues ‘not relevant’ to trials

Mr Jenkins also conceded that once he was aware of the wider issues with Horizon he did not always disclose it in court.

“I believe I did tell the truth and the whole truth as far as the Horizon system was operating in the specific branches at the specific times that I looked at data,” he said.

Asked by inquiry counsel Jason Beer KC why he did not mention information about other faults and defects elsewhere in the Post Office network he said: “I didn’t see whether they were relevant in those particular cases.”

His position appears to be that he was neither truly expert nor fully aware of the grave impact his words and actions could have in criminal trials.

Never told of expert witness obligations

Mr Jenkins insists that at no point was he explicitly informed of the special responsibilities that come with being an expert witness, including those of legal disclosure. He also accused the Post Office legal teams of “putting words into his mouth”.

Ultimately, but too late for many, it was these failures that brought the Horizon prosecutions to a halt.

Legal advice commissioned by the Post Office board in 2013, but not revealed for another five years, concluded that Mr Jenkins evidence was unreliable, and potentially put the organisation in breach of its duties as a prosecutor.

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For Ms Misra, jailed in part on the evidence of Mr Jenkins in 2010, it took 11 years for her conviction to be overturned. Fourteen years on she saw him for the first time since, and watched as he again gave evidence about her.

“At the trial I was really, really happy and really hopeful, there’s an IT expert from Fujitsu coming up and it will be all good,” she told Sky News.

“But he protected the brand and he succeeded in that, and I was sentenced to prison for 15 months.”

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