"I'm so hopeless at all this," mutters Larry Lamb, as he jabs a finger at the screen of his smart phone. He is trying to book a taxi but it isn't going well. Lamb eventually admits defeat and hands his phone to a press officer, who quickly makes the necessary arrangements. He busies himself instead by pouring us both a cup of tea.

"I don't feel like an old man," says Lamb, who celebrates his 70th birthday in October, as he settles into an armchair in a central London office. "But the years are clicking on. I know I'm not going to live forever." Lamb is in a reflective mood. "I never realised that I was going to be old one day. I was about 65 when I thought, 'hang on, this is mortality now, I know there's a limit.'"

Old age and mortality: these are two of the central themes running through Lamb's latest project, The Hatton Garden Job. The film is based on the real-life, headline-grabbing heist, which took place over the Easter weekend in 2015, when a gang of wrinkly criminals with a combined age of 448 stole a reported 200 million of cash and valuables from London's jewellery and diamond district.

The fact that nearly all of the men involved in the "largest burglary in English legal history" were eligible for a free bus pass is played for laughs in Ronnie Thompson's film. "There's old school," jokes one of the thieves. "And then there's just old." But the message couldn't be any clearer: write off these pensioners at your peril. "More and more, the world is run by younger people," says Lamb. "But the way this story is told is an interesting combination of old and young working together."

If that all sounds rather cosy, it's worth remembering that most of the men involved in the robbery were not lovable rogues, as the film suggests, but hardened career criminals. "If anyone wants to consider them as heroes, then that's a personal thing," says Lamb. "They certainly didn't set themselves up to be heroes, this was just another job. I didn't get the feeling that the film glorifies them in any way."

Lamb scoffs, too, when I ask if he ever encountered the criminal world first-hand. "It was so far removed from my life when I was a kid," he laughs. "My family were all east enders but I grew up on the outer edges of London." What about these days? "I've met a lot of them since I've been an actor," he says.

Lamb's career has taken him a long way from those quiet suburbs, most recently when he spent 20 days in the Australian jungle for ITV reality show I'm a Celebrity ... Get Me Out of Here!

After his chivalry and willingness to muck in initially proved a huge hit with the public, things began to go wrong for Lamb when TV presenter Martin Roberts arrived in camp. The pair immediately clashed, with Roberts later accusing Lamb of bullying. Lamb, who was one of the favourites to win the competition, eventually finished seventh.

Lamb waves away any suggestion of animosity with Roberts, though. "Bearing in mind the history of great conflicts on the show, I thought we all got on well," he says. "We all chat to each other. There's a What's App group, so everybody's been keeping in touch. It will go very quiet and then all of a sudden there will be a big flurry [of messages] and all the rest of it."

Did Lamb seek the advice of infamous 2015 I'm a Celebrity ... contestant Lady Colin Campbell, with whom he had a fling in the Seventies? "No, no," he says. "The best thing is to go into the jungle and it's all new."

One of the highlights of the series was the unlikely friendship between Lamb and 26-year-old Gogglebox star Scarlett Moffatt, who Lamb describes as "a dear little thing". But it must be strange, I suggest, for Lamb to see people such as Moffatt achieve, almost overnight, the levels of fame and success it has taken him a whole career to reach.

"When you're in the jungle, who you are comes through very quickly," he says. "And I think that's why [people such as Moffatt] have achieved what they've achieved. They are very straightforward, decent, interesting human beings. It comes right out. They're really likeable people." This sort of generous answer, delivered with complete sincerity is typical of Lamb. He is avuncular, charming and unfailingly polite.

Indeed, the only time he bristles during the interview is when I ask him about his first daughter, who was born when Lamb was 21, and who he saw only once, when she was a baby. Lamb, who has been married twice, now has four children, including the broadcaster George Lamb.

"When her mother got married again, I didn't figure it was for me to interfere with my daughter's new life," Lamb wrote in 2011. "I have no contact with her." But after leaving the jungle last year, he said: "If my daughter or granddaughter were to contact me, then I, and all my extended family, would be thrilled."

When I bring the subject up again, though, Lamb shuts the conversation down brusquely. "I've written about it, you've read about it," he says. "I didn't think we were talking about it. I thought we were talking about The Hatton Garden Job.

"That's been written about by me, it's been written about by a load of other people and I'd be rather surprised that people who read The Daily Telegraph want to understand about any of that when I'm talking about The Hatton Garden Job. Whatever you've read is whatever you've read."

I suggest we talk about something else instead. "Yeah, absolutely," says Lamb, immediately brightening.

Lamb was 27 and working in Canada, initially in the oil industry and then as a construction manager, when he impulsively turned up to an audition for the local theatre company. "I literally walked off the street and that's when I became an actor," he says.

Handsome, self-assured and seemingly irresistible to women, Lamb soon landed a minor part in the first Superman film and a major part in ITV's 1980 gangland drama Fox. This was followed by Triangle, a disastrous BBC soap opera set on board a North Sea ferry, which came third in a poll of the worst television shows ever made. "No such thing as bad publicity," says Lamb. "I had to build a career from zero; I didn't go to drama school. There's no acting in the family, none at all."

In the years after Triangle, Lamb carved out a living with roles in Eighties classics such as The New Avengers and Boon, while landing a series of bit-part roles in shows such as Lovejoy, A Touch of Frost and Casualty. He also had a regular slot on The Bill between 2004-05. His many theatre credits include roles in Educating Rita, A Madhouse in Goa and an RSC production of Hamlet.

Then in 2007 came Gavin & Stacey, James Corden and Ruth Jones's award-winning BBC comedy. It was a late and unexpected break for Lamb, who was 59 when he landed the part of Gavin's lovable dad Mick.

"Gavin & Stacey and EastEnders [in which Lamb played the villainous Archie Mitchell] are turning points in my career," he says. "Literally from the first read-through of Gavin & Stacey, everyone thought, 'wow, this is something.' I'd never really expected myself to be a part of what is now widely accepted as being a comedy classic."

So what of the persistent rumours that the series might be returning? "I can't quite see how it would ever be back on the telly," says Lamb. "But it would never surprise me, bearing in mind the careers that Ruth and James have had, if they might make a film of it. I don't think there's anybody that was in it that wouldn't be up for it."

Before any of that, though, Lamb is heading off on a cycling tour of Britain with his son George for a new Channel 5 series. It seems as good a moment as any to ask Lamb about Brexit. His brow furrows. "I'm a citizen of the world," he says. "I happen to have been born in Britain, but I could have been born anywhere. I just feel that we are part of a greater world and that we are probably better off being allied to our close geographical neighbours.

"All these funny people not wanting their passport to look like everybody else's? I find that beyond belief how people can be so small-minded."

Plenty of life in the old dog yet, then.