A former post office manager, who had to wait 10 years to clear his name, will give evidence at an inquiry into the Post Office's IT system which led to one of the worst miscarriages of justice.

William Graham, once the head of the Riverhead branch in Sevenoaks, was originally sentenced in January 2011.

Between 2000 and 2014, hundreds of subpostmasters and subpostmistresses (SPMs) were prosecuted based on information from the Horizon system, installed and maintained by Fujitsu.

However, in December 2019, a High Court judge ruled that Horizon’s system contained a number of “bugs, errors and defects” and there was a “material risk” that shortfalls in Post Office branch accounts were caused by the system.

Mr Graham was one of 39 people to officially have his name cleared last year, when the Court of Appeal quashed their convictions.

Today, a public inquiry looking into the system's faults and the effect it had on staff, has begun at the International Dispute Resolution Centre in London.

Mr Graham was alleged to have concealed losses which totalled £65,521. He had noticed problems with his accounting system and had already paid £5,000 from his own pocket to conceal an earlier shortfall.

This came after the Horizon IT system installed by the post office, falsely suggested there were cash shortfalls.

Following his conviction, Mr Graham lost his job and struggled to find work with the offence against his name.

The inquiry, which is expected to run for the rest of this year, will look at whether the post office knew about faults in the IT system and will also examine staff shouldered the blame.

Mr Graham, who started the franchise in 2002, began to notice small losses on the books in 2005 and made up the shortfall.

This came around the same time when the IT system was upgraded to an online system in 2005.

Already facing personal financial pressures and having paid from personal funds in 2006, he was accused of making false entries to conceal the losses.

On advice by his legal team Mr Graham admitted two offences of false accounting in November 2008 and January 2009.

The inquiry will also examine whether staff at software firm Fujitsu, which developed the Horizon software to complete tasks such as transactions, accounting and stocktaking, knew the system had flaws while data from it was used in court to convict sub-postmasters.

The judge will also hear evidence on why sub-postmasters and postmistresses were singled out and whether they have been justly compensated, as well as analysing at least 100 written statements.

Mr Graham said: “Even though I’ve been acquitted, it made me feel useless, because I couldn't explain where the losses came from, it made me doubt myself.

“I doubted myself, I felt like my family doubted me.

Mr Graham started private therapy four to five months ago provided by the Post Office legal representation team. He said this was a “massive help.”

“My therapist told me to use Wednesday as a line in the sand moment."

He went on to explain he was ready to put the situation behind him, he said:“It’s now time to forget about it.”

The inquiry's aim is to allow those affected to relay their experiences.

The inquiry's chair, Sir Wyn Williams, said: “I recognise that the task of giving evidence in a public forum about personal experiences will not be easy.

"Without such evidence, however, my ability to reach clear and considered conclusions about the nature and scale of the suffering endured by many over a substantial number of years would be seriously impaired.

"Again, my heartfelt thanks to the persons who will give evidence over the weeks to come.”

Mr Graham is due to give his evidence between on day three of the inquiry, on Wednesday, between 10am and 1pm.