The Great Train Robbery of 1963 remains the most celebrated heist in modern British history, and this week, it has emerged that one of its masterminds was an Irishman whose guilty conscience eventually got the better of him.
On August 8 that year, 15 men wearing ski masks and helmets swarmed onto the Glasgow to London mail train in a sleepy part of rural Buckinghamshire. In an operation marked by military precision, they grabbed 120 bags full of cash and made off into the night with £2.6m in used banknotes, the equivalent to some £50m in today's money. It was the biggest robbery ever carried out in Britain.
Remarkably, instead of drawing the condemnation of the general public - the train driver was savagely coshed with an iron bar - the audacious stick-up was widely applauded for its derring-do, and for giving a bloody nose to a British establishment, whose stock with its people had hit rock bottom.
The train robbery came as the sex'n'spying Profumo scandal, which exposed very low standards in very high places, was hurtling towards its climax with the forced resignation of Prime Minister Harold Macmillan. A fresh wind of change was gathering to blow away the stench wafting from a degenerate ruling class.
This mood for casting off shackles was perfectly captured by poet, Philip Larkin, in his famous lines: "Sexual intercourse began/In nineteen sixty-three/(which was rather late for me)/Between the end of the Chatterley ban/And the Beatles' first LP."
These were strange days, and when the thieves on the run eavesdropped on the word on the street, they found that they were unlikely heroes to a society flirting with notions of civic disobedience.
Two years after the caper, the public approval ratings for the train robbers soared even higher as the ultra-conservative Daily Telegraph carried a piece by the respected author, Graham Greene, asking: "Am I one of a minority in feeling admiration for the skill and courage behind the Great Train Robbery? More important, am I in a minority in being shocked by the savagery of the sentences?"
By that point in time, a caper lifted from the masked-men plot lines of countless old Hollywood westerns had developed into a full-blown reality soap and a cause celebre. Several members of the 15-man gang had been rounded up in a massive police operation. And while the prisoners had widespread public sympathy - or because they had widespread public sympathy - the establishment decided to make an example of them.
Handing out crushing sentences, the presiding judge told the captured robbers that "to deal with this case leniently would be a positively evil thing". He sent most of them down for 30 years.
Not long before, the same judge had reduced the sentence on appeal of one Charles Connelly, who had been involved in a robbery in which a van driver was shot dead. Cutting his term from 15 to 10 years, the judge explained: "The sentence was excessive."
But there would be many more twists and turns, right up to this week.
In 1965, Ronnie Biggs scaled the walls of Wandsworth Prison with a rope ladder and dropped onto the roof of a waiting removal van before making good his getaway to Brussels where he acquired fake identity papers and a new face through plastic surgery.
As the true life drama continued to unfold, and The Great St Trinian's Train Robbery movie send-up starring comic Frankie Howard, went into production, one key figure hid himself away, wrestling with his conscience.
A new documentary opens this weekend in British cinemas which claims that Belfast-born Patrick McKenna, a nondescript post office employee in Manchester, masterminded the Great Train Robbery in cahoots with Ronald 'Buster' Edwards and Gordon Goody. The film is A Tale Of Two Thieves, and the source of the claim is 84-year-old Goody, now an ill man living out his final years in Spain.
For half-a-century, speculation has ebbed and flowed over the identity of the man who provided inside information to the robbers. This shadowy figure was known to gang members as either The Ulsterman or The Irishman, but Goody claimed that only he and Edwards ever saw his face. Even then, McKenna did not reveal his identity, but Goody says he was able to read the informant's name on the inside of his spectacles case.
The Ulsterman not only told Edwards and Goody which train to rob, but he advised them to delay their operation by a day as the August 8 payload would be particularly large. Asked why he'd chosen to finally identify The Ulsterman this week after 51 years of keeping schtum, Goody suggested it would serve as a sort of death-bed confession, remarking: "It's getting near the time when the shop is going to close."
Patrick McKenna is not around to dispute Goody's claim. He died in 1992 of a heart attack, 18 years after retiring into what appeared to be a life close to the breadline.
Speaking this week, McKenna's grandson Mark said: "He was one of those who would sit there at the end of the month comparing his till receipts to his bank statement."
He added: "When mum was younger, they struggled with money. She was walking around with holes in her shoes. Every time he bought a car, it had to be on hire purchase."
McKenna died with just £3,000 to his name. But Goody insists that, in the wake of the robbery, he handed over some £135,000 in two holdalls and a mailbag to McKenna at their final meeting. The Irishman put the loot, worth several million at today's values, in the boot of a grey Austin car and made a clean escape from the biggest robbery in British history.
But Patrick McKenna seemingly couldn't escape his own guilty conscience. At this point in the tale, we're relying mostly on speculation from Gordon Goody, along with the findings of the celebrity people-tracer, Ariel Bruce, who delved into the available details of McKenna's past.
Goody is convinced that the pious Mass-going McKenna couldn't bring himself to revel in his ill-gotten gains and most probably donated his entire stolen fortune to the Catholic Church in Ireland. His nagging conscience possibly salved, McKenna settled back into an anonymous life of raising his family and counting the pennies.
It was left to Gordon Goody to nominate him to crime's Hall of Fame this week with the commendation: "He was absolutely essential to the robbery. It simply would not have happened without him. The information that he provided was spot-on."
For others, the runaway train just kept hurtling down the line as the Great Train Robbery became the saga that kept giving and giving, to investigative reporters, book authors, movie makers and even the pop charts.
Both Ronnie Biggs and Buster Edwards attracted an additional dollop of notoriety when it emerged that their face-changing plastic surgery had been provided by a rogue doctor who'd obliged on-the-run Nazis with the same service in the wake of WWII.
Edwards turned himself in after three years, served some time and ran a flower stall. Phil Collins played him in Buster, the glamourised 1988 biopic, but drink and depression led to a lonely suicide in 1994.
Ronnie Biggs enjoyed an even greater celebrity, flaunting the high-life beyond the reach of British justice in Rio. He sealed his outlaw status by recording a Top 10 hit in 1978 with the Sex Pistols, growling out the vocals to 'No One Is Innocent' backed with Cosh The Driver.
Returning to Britain in 2001 he was slapped back behind bars. Released in 2009, he died last year aged 84.