Jailed Shrewsbury postmistress fighting to clear her name

But she is vehement in her insistence of her innocence.

And she is not alone.

Despite the claims of more than 100 postmasters, the Post Office insists there has been no evidence of any inherent flaw with the Horizon system.

Post Office communications manager Melanie Corfield said there had been exhaustive investigations over the past three years. "Post Office has not been shown any evidence of any systemic problems with the Horizon system," she said.

"The Post Office and independent forensic accountants have found that the majority of losses in cases put forward were, in fact, caused by human errors made at the counter.

"Every case is different and has been individually investigated and independently reviewed. We have gone to enormous lengths to address concerns raised but, many cases are based on allegations which, following investigation, are not supported by the evidence.

"A minority of cases involve criminal convictions. In none of the Post Office's own work, nor through any of the independent forensic accountants' work, has any information emerged to suggest a conviction is unsafe."

However, a leaked copy of the report, which by forensic accountants Second Sight who had been commissioned by the Post Office to carry out a detailed investigation into the complaints, said the Post Office had made it increasingly difficult to progress its investigation.

It also accused the Post Office, which has the authority to prosecute cases on its own, without having to refer them to the Crown Prosecution Service, of failing to meet the requirements of the prosecutors' code.

The report said: "We remain concerned that some of these decisions to prosecute may have been contrary to the Code for Crown Prosecutors with which the Post Office, as a private prosecutor, is required to comply.

"In order to investigate this matter we had requested access to the complete legal files held by Post Office in a number of cases. Post Office has stated that this subjects is outside the scope of our investigation. We strongly disagree with this view."

Referring to the computer system itself, the report said: "As we have previously stated, when looking at the totality of the 'Horizon experience', we remain concerned that in some circumstances Horizon can be systemically flawed from a user's perspective and Post Office has not necessarily provided an appropriate level of support."

The Horizon system has also come under fire from another expert, who helped clear a subpostmaster of false accounting charges 10 years ago. Digital forensics expert Andy Clark said he was shocked the disputes were still going on.

More than 100 others share her story. They were once respected Post Office workers who claim to have fallen victim to a flawed 'Horizon' computer system.

Mrs Shaheen, who ran Greenfield post office in Shrewsbury from 2006, is fighting to have her conviction overturned after being sentenced to 12 months in prison at Shrewsbury Crown Court.

She says the false accounting identified in her case came from glitches with the Horizon system, which records all transactions.

The system has been criticised in a report by forensic accountants Second Sight, which described Horizon as "not always fit for purpose".

The report suggested that errors in the system — possibly combined with raids on Post Office cashpoint machines by criminals using malicious software – were responsible for most of the losses, and that the "vast majority" of the sub-postmasters involved were likely to be innocent.

Mrs Shaheen had pleaded not guilty to the theft of £43,000 after an auditor found discrepancies on the computer system. But on the day of the trial, the Post Office offered to drop the fraud charge if she admitted to the lesser charge of false accounting.

"I was in the canteen at the court when my barrister said the Post Office was prepared to do a plea bargain," said Mrs Shaheen.

"I was given 10 minutes to make up my mind, and the barrister said 'if you plead guilty the chances are you won't get a custodial sentence'.

"He told me that if I pleaded not guilty, and was convicted by a jury, I could get two years."

In December 2010, she was sentenced to 12 months in prison, but was released in March having served three months.

"When I went to court that day, I assumed I would be walking out afterwards, it never occurred to me that I would go to jail," she said.

"The first day was the hardest, I was just going around thinking 'I am innocent, how have I ended up in here?'

"Prison was just like it was on television, it was horrible. The only thing I can say is that I was not bullied. The hardest thing was when friends and family come to visit, and they went home afterwards, but I could not."

She found comfort in the fact that people she knew through her work could not believe she was guilty.

"Customers would write to me and send me little gifts while I was in prison," she added, "and that was one thing that kept me going."

Mrs Shaheen said she and her husband Mohamed had invested £270,000 into the post office and general store when they took over in 2006.

However, after the court case, the business was repossessed without notice, they lost their home and were forced to live in their van.

"They had terminated the post office contract, so I went to work in the shop when I came out of prison," said Mrs Shaheen.

However the loss of income from the Post Office meant the Shaheen's were unable to keep up with their mortgage.

"We had negotiated reduced payments, but one day someone came along and put of a 'for auction' sign," said Mrs Shaheen.

"We asked who had put the business up for auction, and were told it was the bank."

She said the couple were given 10 days accommodation at a bed-and-breakfast in Whitchurch, but were left homeless after that.

"We were living in the van for eight weeks, from February to April," she said.

"It was very cold, it turned nasty in April that year, and we were having to go to wash in the toilets at Tesco or Morrisons.

"One day some residents invited us in for a shower, and eventually we were given a bedsit in Gains Park."

The couple said they first became aware of problems with the Horizon computer system around 2007, when they discovered a shortfall of more than £30,000.

"After four months of arguments with us, they sent somebody to look at the system," said Mrs Shaheen.

Mr Shaheen said that eventually £14,500 of the deficit had been found to be down to incorrect stock records, but the Post Office said we would have to make up the remaining £18,500 or they would lose their contracts.

"At that stage, we though it must have been down to human error, because of a lack of training," he said.

Mrs Shaheen added: "We paid up and thought that would be the end of the matter."

She said one of the problems with the system appeared to occur when they used it to record cash floats which were delivered from the Post Office to the branch so they could make payments to people.

"Say we had a delivery of £10,000 coming in, we would put that into the system, and then it would change to £12,000," she said.

Mrs Shaheen said she became aware of further problems in 2009, and again asked for help.

On September 3 that year, she was visited by an auditor who shut the post office while she carried out her investigation.

"She said there was a discrepancy of £43,000, and rang her manager who told us our contract was being terminated," said Mrs Shaheen.

"We said it must be a problem with the computer, but they said it hadn't happened to anybody else. It was only when we read about Justice For Subpostmasters Alliance later on that we realised we weren't alone."

Alan Bates, chairman of Justice For Subpostmasters Alliance, says problems first surfaced with the computer system when it was introduced more than a decade ago. Horizon is the computer system through which the Post Office carries out and records transactions — from withdrawals from accounts, to road-tax payments and Premium Bond purchases. It works in tandem with a banking system used by the Post Office, through which debit and credit card transactions take place.

But it has been claimed that if these two systems get out of sync, because of a power cut or phone fault, for example – transactions might be repeated or not go through.

This can lead to unexpected losses or gains, and errors in stock records. Sub-postmasters say these are hard to investigate because the system is difficult to use. They also say it is difficult to spot and correct human errors — even involving thousands of pounds. The cash machine system used in the Post Office network has also produced errors, which have then been logged in Horizon.

Many sub-postmasters claim they were encouraged to gloss over losses by the Post Office's own helpdesk, which routinely promised worried callers that errors would iron out over time. Some said they had no idea they were committing an offence by altering their books to ensure they balanced.

Mrs Shaheen praised Shrewsbury MP Daniel Kawczynski, who she said had been making representations on her behalf.

Ironically, Mr Kawczynski had also helped the couple when their post office had been earmarked for closure in 2007, as part of a national scheme to reduce the number of post offices.

"We were the only one of eight post offices in the area who managed to get the closure notice overturned," said Mrs Shaheen.

"With hindsight, it would have been better if they had closed us down."