'So much didn't make sense': The Second Sight investigators who exposed Horizon IT scandal

Forensic accountants Ron Warmington and Ian Henderson exclusively tell ITV News Correspondent John Ray about the "most difficult case" they have ever worked on

They make unlikely heroes: Two men of a certain age dressed in dark blue suits.

Ron Warmington and Ian Henderson are number crunchers. Bean counters. And to hundreds of persecuted people, saviours.

"It’s the most difficult case we’ve ever worked on. But by miles the most important", Mr Warmington said.

Between them, they have investigated multi-million pound frauds and extortion rackets; even given expert evidence in the Wagatha Christie trial.

But nothing compares to their experience with the Post Office.

"It’s when you meet the postmasters," said Mr Henderson. "That’s why you do it. They’re the heart of the community. And they were badly let down by the Post Office."

Mr Henderson and Mr Warmington together are Second Sight; the forensic accountants brought in – reluctantly - by the Post Office more than a decade ago to look for errors in their costly IT system, Horizon.

Ahead of their appearance at the official inquiry on Tuesday, the two men sat down for an exclusive joint interview with ITV News about the case that nearly destroyed them too.

They were threatened by the Post Office with legal action, in an effort to keep their discoveries quiet.

"A post office lawyer was upset with something I said in one of the working group meetings," remembered Mr Henderson.

"He took me to one side and reminded me of my obligations under the non-disclosure agreement.

"And basically, if I said anything out of line that could harm the Post Office, they would not hesitate to take legal action against me. And he said, I would not be able to afford their legal fees. He was probably right."

But Mr Henderson, a former soldier, was not cowed.

The Horizon scandal was brought to the fore by the ITV drama 'Mr Bates v The Post Office'. Credit: ITV

"I carried on regardless," he said. "I've been shot at, I've been blown up. I'm not going be intimidated by threats like that."

The men say the closer they got to the truth – that the Horizon system was deeply flawed and that consequently the convictions of hundreds of sub-postmasters were unsafe – then the more the Post Office sabotaged their investigation.

"They were limiting access to key documents. Key files. Failing to answer questions," said Mr Henderson.

It became clear the client was the also the culprit.

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"They didn’t want us to do the job that we were appointed to do; which was to seek the truth, regardless of the consequences," he said.

Their repeated warnings to the Post Office were ignored and Mr Warmington took the decision to secretly tape conversations with senior executives.

"I no longer trusted them," he said.

He thought long and hard about quitting the investigation, only to be talked out of it by Post Office campaigners, who told him: "If you pull out there will be nothing between the wolves and the post masters, so please don’t pull out", he recalled.

In the end, the decision was made for them and they were dismissed by the Post Office.

Paula Vennells was stripped of her CBE back in February. Credit: PA

More than a decade on, Paula Vennells - the then-chief executive - began her evidence to the official inquiry with an apology to "Ron Warmington and Ian Henderson for making their work harder."

The men have waited a long time to deliver their verdict on her.

"I'm certain that she knew," said Mr Warmington, "but I think her brain seems to be working in a way that allows herself to know something, and then to dismiss it from her mind.

"What I saw in Paula and many others was this compartmentalisation where despite the clarity with which we'd expressed the problems, about the prosecution practice and poor investigations and so on, she seemed to be able to forget that.

"I think it was displaced by advice that she obtained from lawyers."

Ms Vennells has denied wrongdoing. In her three days of evidence to the inquiry she claimed she too had been misled by her staff.

Mr Henderson and Mr Warmington want a prompt police investigation to pick up where they left off all those years ago.

What is clear is that if the Post Office expected a whitewash from Second Sight, they picked the wrong guys.

"My parents ran a Post Office for many years," Mr Henderson revealed.

"And very early on I realised we were looking at miscarriages of justice and misconduct by senior people within the Post Office and I wanted to expose that."

"There was just so much that didn’t make sense. I mean why would hundreds of sub-postmasters go rogue and start stealing. It was implausible."

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