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The Post Office was "constantly sabotaging" the work of independent investigators probing issues with the Horizon IT system, an inquiry has heard.

Forensic accountant Ian Henderson said the Post Office unjustifiably withheld documents from his company, Second Sight, which was brought in to investigate the accounting software in 2012.

He said protecting the Post Office brand was the priority, rather than supporting sub-postmasters.

Mr Henderson added that former Post Office boss Paula Vennells tried to steer investigators away from looking into potential miscarriages of justice.

Between 1999 and 2015 the Post Office prosecuted hundreds of sub-postmasters for offences such as theft and fraud on the basis of faulty accounting data from the Horizon IT system.

In 2012, under pressure from MPs, the Post Office commissioned a report from Second Sight to look into claims from sub-postmasters that Horizon had been to blame for shortfalls in their accounts, rather than criminality.

Mr Henderson and his colleague Ron Warmington began investigating various cases but were sacked by the Post Office in March 2015.

In a statement to the inquiry into the scandal, Mr Henderson said that rather than being interested in getting to the truth of what happened, the Post Office had tried to obstruct Second Sight's efforts.

"Requests for documents were either ignored or responses were excessively delayed," he wrote. "Unjustified claims of legal professional privilege were used to justify withholding documents from us."

As part of the probe, Second Sight asked for documents relating to prosecution cases to be collated.

Mr Henderson said that "within days" of being provided with the available documents in late October 2012, "we realised that we may be looking at a significant number of miscarriages of justice".

'We were dealing with a cover-up'

However, he said that Ms Vennells, who was Post Office chief executive at the time, "frequently and consistently attempted to steer Second Sight away from investigating potential miscarriages of justice".

By February 2015, Mr Henderson said he "felt we were dealing with a cover-up" by the Post Office "and possibly a criminal conspiracy".

At the time he was concerned the Post Office would take him to court for alleged breaches of confidentiality and a non-disclosure agreement.

His partner Mr Warmington said it became clear that the Post Office was "aware, possibly at the highest levels" that the Horizon system had "for years been producing spurious discrepancies in branch accounts".

He also said the Post Office "had been responsible for numerous unsafe prosecutions, convictions, custodial sentences, bankruptcies and even suicides" due to the "improper behaviour" of its prosecutors.

The investigators wrote an interim report into the Horizon system, published in July 2013, which identified bugs that could have made Horizon convictions unsafe.

Following this report the Post Office, along with Second Sight and campaigners led by former sub-postmaster Alan Bates, set up a mediation scheme in August 2013.

The Post Office closed this scheme in March 2015.

Mr Henderson said that towards the end of the scheme "some questions asked 12 months earlier had still not been answered".

"Protecting 'the brand' was the priority, not supporting sub-postmasters," he said.

He added that many aspects of the individual cases that were prosecuted "just didn't make sense".

"For example, in none of the cases that we looked at did we find any evidence of personal gain or benefit," he said. "This may indicate that the alleged loss was not real and was more likely to have been caused by a faulty computer system."

The Post Office did extensive vetting of sub-postmasters before taking them on, he added, but "would have us believe that significant numbers... had suddenly become career criminals. I found this implausible."

Mr Henderson added that at one point the Post Office had more people working in its public relations department than its legal department, which appeared an "inappropriate" and "unsustainable" priority for the business.