Taro Naw investigates the fate of some of Britain's postmasters

As former postmaster Noel Thomas, from Anglesey, continues his battle to prove he was wrongly imprisoned three years ago, Taro Naw, Tuesday 8 September, BBC Wales on S4C, reveals new evidence supporting his claims that other postmasters faced the same fate across the United Kingdom.

Clwyd West MP, David Jones, who has a similar case within his own constituency, says: "I'm sure something's going on and the Post Office need to answer the questions that these people are asking. I am going to ask for a debate at the House of Commons."

Back in 2005 in Gaerwen on Anglesey, postmaster Noel Thomas found that his weekly computerised accounts were showing takings higher than what was actually in the till. He says he never stole a penny and that the computer – one used by the Post Office across Britain – was faulty.

Even though he rang the computer helpline several times and asked the Post Office to research the matter further, he says the response was lukewarm until the mis-balance reached £48,000 and a financial audit was taken. He was arrested and accused of theft and false accounting, and was jailed for a year.

"I lost some friends," said Mr Thomas, an ex-county councillor.

"My image was in tatters and I lost a lot of respect and that was a lot to deal with. But that's when you find out who your friends are, when you are in trouble."

He had given up proving his innocence until he received a phone call from England last year from Roch Garrard, who said the same thing had happened to his local postmistress in South Warnborough, North Hampshire.

Jo Hamilton runs the village shop and she was also postmistress until she started finding unexplainable anomalies in her accounts back in 2003. As in Noel Thomas's case, the Horizon system was claiming a balance from week to week that was higher than she had taken. She rang the helpline several times and found that at times their advice increased the deficit.

"I didn't have any confidence in the helpdesk and I didn't know where to turn for help so I, like an idiot, kept signing the accounts even though I knew the money wasn't there," she says.

"But I knew I hadn't taken anything and I couldn't open up the next day unless I had signed the accounts as being correct. But I paid the price for it eventually."

After being prosecuted for false accounting she had to pay the Post Office £36,000 although she says she never took a penny and that it was the computer system's fault. Her local villagers were so convinced of her innocence they raised £9,000 to help her pay the Post Office.

Retired senior probation officer Roch Garrard had done some research before he rang Noel Thomas and found many others like him and Jo Hamilton.

"I'd found a whole lot of people like Jo – middle aged, middle class, never put a foot wrong, perfect characters, often they'd been sub-postmasters for a long time all of whom suddenly turned into these criminals," says Mr Garrard.

"It didn't make sense to me so I started to contact some of them and said to them – 'This is what happened to our postmistress, what happened to you' – and the stories were all so similar that I thought there must be something wrong."

Tonight Taro Naw will be revealing similar cases all across Britain – ex-postmasters that have lost their livelihoods, all claiming the computer system was at fault – and some that still work for the Post Office who are correcting the mis-balances by paying the difference from their own pockets to avoid the same fate as the others and to keep their business afloat.

The Post Office denies there are any problems with its computer system saying it is robust and deals with millions of transactions every day and that they have won several court cases proving this.

But how then can they explain all these similar cases in villages, towns and cities across Britain?

Taro Naw, Tuesday 8 September 2009, BBC Wales on S4C, 9.30pm