The quiet Great Train Robber reveals identity of the gang's mystery insider

Now 85 and living in Spain, Douglas Gordon Goody has decided to share his story with the world – including unmasking the mysterious 'Ulsterman' who helped to plan the crime – then vanished

He has kept his secrets for more than 50 years: the quiet man of the most infamous criminal gang in British history, both mastermind and instigator of the Great Train Robbery.

The Rolex has been replaced by a Swatch and the white Jaguar and sharp suit are long gone. So is the equivalent of £2.5m that was his share of the crime. Now aged 85 and one of just two surviving members of the 15-strong gang, Douglas Gordon Goody lives quietly in the Spanish countryside with his partner, Maria, and their five dogs. It is back to his rural roots for a man whose introduction to crime was smuggling cattle over the Northern Irish border to dodge customs.

Two key facts about the audacious 1963 robbery that gripped the public imagination still remain unknown – who was the insider, the mystery man who fed the gang the information they needed to know to target the train? And who was the gang member who bloodied what was planned as a "gentleman's crime" by coshing the train driver?

Goody is clear on the first. The Observer can reveal that Patrick McKenna was the "Ulsterman". Then a 43-year-old Belfast-born postal worker in Islington, north London, he was introduced to Goody via a third party. They met four times in early 1963 and McKenna told them how a post train operated, enabling the gang to successfully stop the Glasgow-to-London Royal Mail train in the early hours of 8 August 1963, getting away with more than £2.6m – the equivalent of £48m today – all in £5 and £1 banknotes. It was McKenna who told the gang to change the original robbery date from the 7th because the next day's train would be carrying more cash.

Goody only knew the man's name because he happened to pick up his glasses case, which had fallen in the grass when McKenna went to buy ice-cream for the plotters on a warm May day in Kensington Gardens, and saw it scribbled inside. The name and the fact that, as Goody later recalled, McKenna looked like the comedian Frank Skinner were all Goody knew, but now a documentary team have used those clues to trace the Ulsterman.

But while there is now a face and a name to the insider the police never found, as one mystery is solved, another deepens. After McKenna picked up his share in two holdalls and a mailbag and piled them into the boot of a grey Austin 10, the money disappeared from history.

His widow and children, who live in Manchester, were "shocked and flabbergasted" to hear about McKenna's part in the conspiracy, says Simon Howley, the producer of the documentary film A Tale of Two Thieves. A quiet church-going man, who lived a simple life, continuing in the post office until his retirement, McKenna never owned a car – and always worried about the security of trains. Howley suspects the money could have been stolen from McKenna. The family think his conscience may have got the better of him and he donated it to the Catholic church he frequented. He certainly didn't spend it.

For Goody it's the release of a long-kept secret. In fragile health, he has finally decided to write a book, joining several others of the gang who have written or been the subject of books, films and documentaries. "I've read them all, I watched the film Buster. It was good. Everybody loved Buster."

Goody was sentenced to 35 years and served 12. He insists that while the robbery was "my job", evidence at his trial was faked. He was arrested in Leicester, where he had been staying in a hotel with his then girlfriend, a former Miss Great Britain. "The receptionist thought I looked like the photofit they were running in the paper of Bruce [Reynolds, his co-gang leader] and she rang the police."

A pair of shoes marked with paint from the hideout farmhouse was the key piece of evidence. "They weren't the shoes I'd worn for the train. I wore desert boots. They took my brown suede shoes from my mum's and they appeared at court, complete with yellow paint. The judge knew I'd been fitted up. But I had the worst record of all of them. He said I was the saddest case in front of him, and said my powers of leadership would have won me medals in a war. Then he pulled a 30.

"It's a fearsome thing to be going to prison with a long sentence. It's shit and sugar. There's a way of doing time, you either lie on your bed and cry or you go to the gym and the library." Goody taught himself Spanish and read extensively, but also considered escaping a few times – as two of the gang, Ronnie Biggs and Charles Wilson, did. A change in the law saw him released early in 1975.

"I knew if I stayed in England with my pals I'm going to go at it. So I thought 'no', and here I am. Don't ask me the question would I do it again. People do ask. If I was 33 and the facilities were the same as they were then I'd be tempted but present-day crime is not for me. Not really.

"The fact the driver was hurt, that's the thing I regret. Nobody was going to get hurt. I was always a gentleman, we were gentleman robbers. When Jack Mills fell and hit his head we all looked out for him, Charlie bandaged it up for him and Tommy gave him a cigarette and sat with him. We knew it changed everything. I was choked, choked."

It is generally agreed it was either Buster Edwards or James Hussey who coshed Mills, who died of leukaemia in 1970, and Hussey made a deathbed confession in 2012. "It wasn't Hussey," said Goody firmly. Leaving Buster? A lifetime of omerta, a code of silence among thieves, runs deep, and he shrugs uneasily: "You might take your pick," he says.

He is also uneasy at the mention of Ronnie Biggs. "Biggsy was an arsehole. I didn't like him, no one did. His one job was to bring a train driver and he brought a guy who couldn't do the job. They had to go and sit in the car. Even the ones who pulled him out of prison had brains way out of Biggsy's league."

Many see the train robbery as the end of an era, the last unarmed cash robbery before guns and drugs began to dominate British crime. Goody also believes the harsh sentences – controversial even at the time – ushered in a change.

"Guns came in after the train," he says. "I owned two shotguns but I'd never have taken them on a job. If someone is on a bit of work and they're going to do a lot of bird, then they've nothing to lose. They made a rod for their own backs."

Goody doesn't return to England, or to Ireland where he was born, but his memories are sharp, his language caught in a timewarp of 60s Britain. "My old man wanted me to be a plumber's mate. I wanted to be a criminal. You never have to work hard to be a criminal. You do something, see something, you've no money, you take it. It gets easier next time and then you don't feel no qualms about what you've done. Then you're a thief. I was always a thief, never a gangster."

Like most of his gang, much of his money was spent by the time he emerged from prison by the best friend he'd left it with and he is tight-lipped about what is left. But he remembers well what it looked like, as the gang split it into shares in their hideout. His eyes mist as he describes the floor-to-ceiling piles of banknotes. "It was a sight to see. I wouldn't mind seeing it today."

His book will be out shortly. "I suppose it was time to put a few things straight. I keep being called a hairdresser – I was never a hairdresser!" He insists he never talks about the robbery to his partner, and although his daughter is "proud of her dad", he has made his grandson a poster for his room:"Don't ever try to imitate your grandad," it says.

The Great British Train Robbery: A Tale of Two Thieves will be in cinemas on Friday, and on DVD, Blu-ray and online from 6 October. It is produced by Simon Howley of Kowalski Media and directed by Chris Long, producer of The Mentalist