A post office owner serving a life sentence for murdering his wife hopes to submit a fresh appeal against his conviction, claiming the discredited Horizon computer system was used to wrongly frame him for the killing.

Robin Garbutt has maintained his innocence ever since his wife, Diana, was bludgeoned to death in their flat above the post office in Richmondshire, North Yorkshire, in the early hours of 23 March 2010. Many of his former customers still visit him in prison regularly, refusing to believe he was capable of the brutal murder.

Some were served by him at the Melsonby Village Shop and Post Office on the morning of the killing and reported him to be his “normal cheery self”. They cannot countenance the idea he happily took their money for the morning newspapers or cigarettes within a few hours of beating Diana to death.

But Diana’s mother, Agnes Gaylor, said: “It’s obvious to anyone that Robin is taking advantage of the Horizon scandal to gain publicity. I don’t believe he’s going to get far with it.”

Garbutt has always insisted a man wearing a balaclava and holding a gun made him hand over £16,000 cash, claiming they “had” his wife. He said he handed over the cash and then ran upstairs, to find Diana dead.

He was found guilty at trial in 2011 on the basis of circumstantial evidence, with the jury split 10-2, a majority verdict.

With no DNA evidence to link him to the murder or the metal bar used to kill Diana, Garbutt was convicted in part after the jury heard evidence from a Post Office investigator using data from the Horizon system. This purportedly showed he was stealing money from the Post Office and then killed his wife to cover up his theft.

The prosecution claimed Garbutt concealed his theft via a series of false declarations about the amount of money in the Post Office safe. The investigator told the jury the pattern of high overnight cash declarations was one “I have seen replicated across many Post Office Limited fraud cases in the past”.

The jury was told he may have staged the armed robbery to cover up the losses, killing Diana either to make it look “real” or because she had found out what he was doing and he needed to silence her.

A second possible motive was aired in court relating to Diana’s relationships with other men. In June 2009 she had joined a dating site, Badoo, to chat to men, and had been actively receiving messages up until her death.

The couple had been to marriage counselling and Garbutt told the jury that his wife had a higher sex drive than him, but that their problems were in the past. “We were as close as close can be,” he told the jury.

Three men were called to give evidence about their relationships with Diana while she was married, all apparently ending at least a year before the murder. One, who used to go on long bike rides with her, said she was thinking of renting a place in the village to give her and Garbutt both space to “think”.

Garbutt was given a life sentence with a minimum tariff of 20 years and will not be considered for release until October 2030.

He lost his first appeal against conviction in 2012 and has been knocked back three times by the Criminal Cases Review Commission (CCRC), which can refer cases back to the court of appeal.

Answering questions from HMP Wealstun, a low-security jail in West Yorkshire, Garbutt said he hoped the government’s decision to quash all Horizon-dependent convictions could pave the way for him to make a fresh appeal.

He said he had been watching the ITV drama from his cell and found it “deeply upsetting – I felt for every person affected”.

He stressed that his case was not the same as theirs, but said: “The way the Post Office dealt with me was the same way that they dealt with the other people.”

He said unreliable data from Horizon “was used in court to make me look bad”. He claims he and Diana often could not reconcile the fortnightly balances with the figures on the computer, so would put cash in themselves if there was a shortfall or take it out if there was a surplus. “We had a lot of problems with the system going down, not working,” he said.

“All the [financial] evidence used against me at trial, we have no means of checking whether it was accurate.”

The jury in his trial were shown financial records from the 15 months leading up to the murder and two Post Office experts said the branch was requesting unusually large amounts of cash over a period when it was not audited. After his conviction, Garbutt was able to obtain records dating back six years, including audited records, which he claimed showed “there was nothing unusual” in the cashflow in and out of the post office when Diana was killed.

These new records were submitted to the court of appeal in 2012, but the appeal was rejected. The judges noted that the evidence of “financial strain” was not limited to the post office: the couple had £44,000 of debt on personal credit cards and an overdraft.

The prosecution case was that Garbutt killed Diana as she slept and then opened the shop as usual at 4.30am, pretending everything was normal until just after 8.30am, the earliest time the post office safe could be opened because of its timed security system. The jury was told that within two minutes of the safe being opened, Garbutt had dialled 999 claiming to have been robbed and to have found his already dead wife.

The jury was told he had not activated the silent panic alarm, despite the business having been held up in another armed robbery almost a year previously, in which £11,000 was taken. No one was ever charged in connection with that incident.

Garbutt told the jury the armed robber must have snuck into the unlocked flat while he was unloading stock from his car in the early hours, killed his wife quietly and then lain in wait for him to open the post office safe and run off with the money, slipping away unseen during the morning rush hour.

He said the Horizon data was just one piece of evidence which could not be relied upon. North Yorkshire police officers lost key evidence, including a clump of hair found on the pillow in the couple’s bedroom. The hair, which was a different colour to both Diana’s and Robin’s, was never taken for forensic analysis and was lost.

A gun and balaclava were found behind a pub in Thornaby, 30 minutes away from the post office, but police quickly decided it was irrelevant. The murder weapon, a metal bar, was only found several days after the murder, on top of a high wall nearby. The jury heard it was contaminated by the police officer who handled it, and may have sneezed on it.

Garbutt claims that unbroadcast footage filmed by Tyne Tees television the day after the murder showed the weapon was not there, meaning someone put it there later, when he had gone to stay with his sister many miles away. There is also contested pathology evidence about the time of death relating to the stage of digestion of the fish and chip supper in Diana’s stomach.

Edward Abel Smith, an author researching the murder for a book, said: “Although I do not have a strong opinion either way, there is ample evidence to suggest Robin Garbutt’s conviction is at least unsafe. North Yorkshire police have a lot to answer for with their investigation, which involved losing and contaminating evidence.

“The jury were told that Robin Garbutt had been stealing from the Post Office, and that his transactions followed a similar pattern to other Post Office fraud cases which we now know relied on faulty data from the Horizon system. Those convictions have this week been declared unsafe and are going to be quashed. Shouldn’t Robin now get a fresh, fair hearing in front of a new jury?”

The Department for Business and Trade, which oversees the Horizon compensation scheme, said: “We will be unable to comment on this as it is an individual case.”