When the Hatton Garden Safe Deposit Company, in the heart of London's diamond quarter, was ransacked in April 2015, the fugitives fled with up to £200 million in diamonds, gold bullion, jewellery and cash.

It made the Hatton Garden heist the biggest-ever burglary in English history. It was also one of the most ingenious, with the perpetrators abseiling down a lift shaft before using an industrial Hilti core drill to tunnel through the vault's nearly two-foot thick walls, to bypass security and the main vault door.

Immediately, news reports were rife of Russian Mafia or Albanian gangsters assembling the creme de la creme of the criminal underworld to pull off such a spectacularly brazen break-in at Easter Weekend. But, when the truth eventually emerged, it was stranger than fiction: the actual culprits turned out to be a motley bunch of retired crims and old-timer gangsters from London's East End, in their 60s and 70s.

"Yeah, everyone thought it was the Russians, or an elite Eastern European gang because it was done so cleverly," nods Ray Winstone, who plays one of the younger members of the gang, 60-year-old Danny Jones, in King of Thieves. "So, when it came out that this was done by a load of old Cockneys, it was like; 'How the bloody hell did that happen?' No one saw that coming."

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"Oh blimey, the story of these old guys doing that, you wouldn't Adam and Eve [believe] it," hoots Michael Caine, in playful Cockney rhyming slang that echoes his character, Brian Reader, the 76-year-old ringleader and mastermind behind the plot.

"If it wasn't a true story you couldn't imagine anyone writing it, because people would say it's so ridiculous. But that's what makes it so funny, what makes people laugh because you couldn't make this up."

It's true. In fact, when the identity of the Hatton Garden thieves surfaced, the gang, dubbed the "Diamond Wheezers" was lauded by the press and public for their old-school, British industry – despite the seriousness of their crime, because they proved that you don't need to be a spring chicken to pull off one of the biggest heists in history.

Unfortunately, being seasoned, veteran criminals more used to burgling in the 80s and 90s meant that they weren't as clued-up on modern technology, CCTV or internet search histories either and they were quickly under suspicion, then surveillance.

In fact, many of the hilarious, pithy and puerile banter, one-liners and insults in King of Thieves are directly lifted from their Friday night drinks down the pub, all recorded on CCTV or hidden wires, taping their conversations.

Some of these exchanges were so rough and raw, points out director James Marsh, that he toyed with the idea of toning down the language for Australian and New Zealand audiences.

"He did talk about that and we ended up having to go in and do some overdubs to change some of the swear words to 'flip' rather than 'f***," acknowledges Winstone. "But that was more for the aeroplane version than Australia or New Zealand, I think. I mean, tone it down for Australia? Nah! Maybe for New Zealand, because they're gentler..."

"I wanted them to change some lines," admits Caine. "I had to say the 'C' word in this movie, which I've never said in a movie before and I didn't like it. So I asked them to take it out, but they wouldn't.

They said that he [Brian Reader] had said it and was the sort of person who says that, so I had to stay with it."

It's a surprising confession from Caine, who's renowned for his colloquial Cockney depictions, his rough, tough gangster roles, like in The Italian Job and Get Carter, and straight-talking parts in The Cider House Rules and now King of Thieves. But then he's full of surprises, like when he talks about how he first got into acting.

"I was in a youth club and I played basketball, because I was tall for my age," he reminisces. "One day, I peered into another room, through the glass door and noticed all the prettiest girls in the club were in there. So I asked the vicar, who was running the club, why all these girls were in there and he told me it was the drama class. From that point on, I would stop and peek in. One day I was leaning on the door, looking through the window, trying to see a girl I really fancied, and I fell in.

"The woman running the club said; 'Come in, we've got no men' - because none of the boys would join the class as being an actor was regarded as effeminate and associated with homosexuality in those days," he explains. "But when I was in there, looking at the girls, I thought: 'Well, I'd rather do this than do basketball.' So, that's how I became an actor and how I got to kiss the girls."

That was Caine's last acting slip-up and since then he's become one of the most-respected and revered British actors in history, praised for both his eccentric English parts and plain-speaking, working-class portrayals. But having grown up in South-east London, in the 50s and 60s, then an impoverished hotbed of criminality and gangster culture, was there ever any chance of him ending up like his character, in King of Thieves?

"No, never," insists Caine. "I understand him because I grew up with loads of Brian Reader types, all around me. Even some of my relatives were Brian Readers – they weren't that honest - so I understand what he did and why he did it. But I would never do it myself.

"We were a completely honest family," he adds. "When I was growing up and people around us went to prison, for robbery, my mother would always say to me: 'You're too proud to ever want anything belonging to someone else, get it yourself.'" And she was right. I'd rather earn it, honestly, than take it. That's why I've never stolen anything, from anyone."

LOWDOWN:
Who: Michael Caine and Ray Winstone
What: King of Thieves
When: In cinemas today