Last May, Dudley Higgins was convicted of a crime he did not commit. A month later, inside prison, he met the man who did do it. Bob Woffinden investigates

'It was Euro 2000 at the time," recalls Dudley Higgins. "We were just standing around outside the post office, discussing the matches and all that. The next thing I know, two plain-clothes officers came up, flashed their badges and said they were arresting me on suspicion of robbery. I said, 'Are you sure?' They said, 'Yes, you - you with the round, chubby face.'"

So, on June 26 2000, Higgins was arrested by officers from the West Midlands police force in connection with an £8,000 robbery at a sub-post office; the very one he had been calmly standing outside for the best part of an hour.

The police charged him and confiscated his car, alleging that it had been purchased with proceedings from the robbery. When the case went to trial in Birmingham in May 2001, the jury unanimously convicted him. Higgins received a five-year sentence and was taken to Winson Green prison.

He had been there less than a month when he bumped into a new inmate on the corridor. Mark Vendryes was a huge man, (1.93m) 6ft 4in, and very heavily built. He immediately told Higgins that he had carried out the robbery of which Higgins had been convicted. In fact, Higgins had already heard that through the grapevine. However, it was a complete surprise when Vendryes said that he had carried out a second robbery at the same post office exactly two weeks later.

"He said, 'They know I robbed it twice,'" recalls Higgins. "He told a probation officer, he didn't hide it, soon everyone knew. He was apologetic to me - he knew that I wasn't that type of person."

Higgins passed the details of the conversation on to his lawyer, Maslen Merchant, and Merchant's subsequent inquiries began to unravel another strange episode in the chequered history of the West Midlands police.

Higgins, too, was a large man, and there were those who said that he looked like a big, round football. Now 33, he came to this country from Jamaica. At first, he stayed with his brother in the Handsworth district of Birmingham, but then found a flat for himself and his partner, Pamela, in Newtown. Their children were born in 1997 and 1999. He worked initially as a washing-machine engineer, but by the time of his arrest, he was employed as a security guard for a company run by a local magistrate.

"When I was arrested, I was taken to Steelhouse Lane police station. They asked if I wanted a solicitor. I told them, 'For what? I don't need no solicitor. What I tell you is the truth.' When they charged me, I said: 'You've got the wrong person. How can I live 75 metres from the post office and rob it? I'm not crazy.' They said: 'It's you - you with the round, chubby face.'"

Higgins had never previously been in trouble with the police, either in Jamaica or England. He spent two nights in custody, before his solicitor got him bailed. Immediately, Higgins dug out all the documents relating to the entirely legal purchase of his Honda Accord, and managed to get it back.

The prosecution case depended exclusively on identification evidence. The post office manager, Andrew Gilling, and another witness had "identified" him as the robber as he stood outside the post office. Gilling even provided a drawing of the robber that he had made immediately afterwards.

A third witness picked out Higgins in a video identification parade, an increasingly common feature in robbery investigations. Instead of combing the local streets for people of similar appearance to the suspect, police can simply put him up on video alongside a gallery of images drawn from the appropriate video bank. However, Marcel Berlins has already commented in the Guardian that this system will incorporate all the failings of the old one. In view of Higgins's unmistakable appearance, questions remained about the fairness of this video parade.

"At trial, they were fishing," recalls Higgins. "They even bring up my car again, but they didn't have nothing: no fingerprints, no forensic, nothing, just one chubby face. I got five years. I thought the world was caving in on me."

Giving evidence, Higgins said that there was talk on the streets that the man who had done the robbery was hiding out in Wales - "but the prosecution told the court, this man does not exist". Far from not existing, Vendryes appeared before Higgins in prison just a few weeks later. He had been arrested in Cardiff and transferred to Winson Green.

In preparation for his appeal, Higgins sacked his trial lawyers and hired the highly recommended Merchant, whom he told about the meeting with Vendryes. At trial, the prosecution had mentioned a second robbery, but had not disclosed any paperwork in connection with it. "We went back to the police," says Merchant, "and asked for all previously undisclosed material in relation to the identification of Higgins by Gilling."

As a result, Merchant realised that Higgins's own information was spot-on. There had been two robberies at the same post office by the same man, on April 15 and 29 2000. Immediately after being robbed for the second time, Gilling had made another statement about the robber: "I immediately recognised him as [the man] who had robbed the post office two weeks earlier and threatened me with a crowbar. I was terrified. I was in shock. I couldn't believe it was happening again by the same person. He said, 'Don't give me any of those forged notes as last time.' I am in no doubt that it is the same person who was involved in the last robbery."

Nothing could have been clearer. Having had the dubious benefit of seeing him twice, Gilling then made a drawing of the robber.

However, by the time the case went to trial, this second robbery had been airbrushed out of the picture. The investigating officer, DC Patricia Mooney, withheld Gilling's vital April 29 statement, and even denied it existed, allowing the crown to extract a formal admission to that effect from the defence. Then, in order to make it appear as though the drawing made after the second robbery was instead made after the first, Mooney had changed its exhibit number (from AJG3 to AJG1).

When the case went to appeal, in January this year, the judges said that there were matters that caused "extreme concern", and condemned the actions of the police in what, for the court of appeal, were unusually forthright terms. Stressing that Mooney had been "responsible for and had full knowledge" of the inquiries into both robberies, they concluded that the key statement of April 29 was "clearly suppressed". They added: "It defies belief to suggest that she would not have realised the significance of the material."

Noting that Higgins had previously been of "good character", the judges quashed the conviction and set him free. They commented that the way the case had been investigated made it impossible for the crime ever to be resolved. Finally, they said that they would ensure that their judgment reached Sir Edward Crew, chief constable of the West Midlands police, "so that he can make such inquiries as he wants to ensure that the members of his force never conduct another trial in the way this one was conducted."

A spokesman for West Midlands police said: "An internal investigation supervised by the Police Complaints Authority is under way, and we are unable to comment further."

Higgins, who lost six stone in prison, says: "Deep down in my heart, I can't believe it all. Even now, if I see a post office, I'm scared to even walk past. I'm still getting flashbacks. Just talking about it all to you stresses me."

Vendryes is serving five years for other armed robberies. Mooney is on maternity leave.