He is huge enough to be able to turn down a reported pounds 500,000 for a day's work on an advert for Typhoo, and a project with his name attached will get green-lighted before you can say "record ratings". His Darling Buds of May was the first programme in history to reach the top of the charts with its very first episode. "There isn't anybody more popular," is the assessment of Martyn Auty, Jason's producer on ITV's A Touch of Frost, the high-class detective series which returns tomorrow. "The sort of figures he got for the last Only Fools haven't been seen since the early 1970s, when there were only three channels."
Only half-joking, Jason tells me during our interview that "Even now the chief executive of Yorkshire TV is waiting outside to twist my arm up my back about certain things," before adding, as if to emphasise his clout, "but I'm not allowing him to make me do something I don't want to do. Good parts just don't fall off trees. I try to be very careful about what sort of projects I attack. There's an audience out there that expects high standards from me."
So just how has he reached such giddy heights? Why does he attract viewers like moths to a flame? Up close, in a dark check jacket, and blue shirt and tie, he looks competely unremarkable, like an Ordinary Joe - which may well be the secret of his success.
Obviously uncomfortable with publicity - a virtue, some would say- he nevertheless flashes the odd Del Boyish twinkle once he loosens up. Belying his own description of himself as "thick", he possesses a native wit and a wicked sense of humour. Pre-empting questions about the distinguished grey beard he is currently sporting, he quips: "I've grown it because I'm looking forward to getting the smaller parts that Sean Connery is going to turn down. Mr Connery does turn them down, and there's a smaller, uglier version ready to help him out."
Auty reckons that "He has great empathy with characters. The audience think that he's just a regular guy, the bloke who lives down the road, and forget that he's an actor - which is the best definition there is of being a supreme actor. You don't sit there watching a performance, you have a window onto a character called Pop Larkin or Jack Frost. You start to live the whole experience with him."
Malcolm Bradbury, who wrote screenplays for Jason with A Touch of Frost and Porterhouse Blue, comments on "the skill with which he takes on the next thing. You get used to Del Boy in Only Fools and Horses or Granville in Open All Hours. But then it turns out that he can do Skullion in Porterhouse Blue or Pop Larkin in The Darling Buds of May. He has enormous range."
But fame, as the showbiz cliche would have it, has come at a price. Fed up with people crying "luvvly-jubbly" at him in the street, Jason is rumoured to sometimes leave his home near Aylesbury in disguise. Much to his annoyance, he was pursued on a recent diving holiday in the Caribbean by some particularly tenacious tabloid journalists. With an understandable hint of testiness, he says: "If you read the Express, you'll find out exactly where I was. Don't talk to me about people who chase you halfway round the world just to get a photo of you. Leave me alone. Let me have a holiday."
It's not as if he takes that many. He is famous for his devotion to work, revealing that "I rarely go out, and I am not interested in golf or anything like that. Being an actor is like being a monk. You have got to be dedicated."
On Frost, Auty says, "David never accepts a script at face value. He worries away at every line just to get that sense of an ordinary guy doing what David describes as an extraordinary job. As an actor, he follows Stanislavsky - though he'd chide me for that and say, `Stan who?'. He thinks what the character would have for breakfast and what clothes he'd put on - all the things that make him more credible. I have seldom seen an actor so immersed in his character. He's a perfectionist in the right sense of the word because he'll never accept something as just OK; it has to be good."
Jason has relished the role of Frost. When he was offered the part of the moody detective, he explains, "It was the first time someone had thought that I could possibly show I'm more than just a comic actor. People tend to feel that if you're a comedian, you can't act. It was a big challenge, but you've got to stick your neck out. Unfortunately, when you do, it tends to get chopped off."
Jason has never lost his head with Frost. The series has prospered partly because it has eschewed the full-on sex and violence that is so fashionable with some cop shows. "You can get your message across without having to be graphic," he reasons. "We're ageing primates, and we can't all be seen jumping around like Tom Hanks."
It is the very down-to-earth likeability of that statement which illuminates his performances. But after so many sheerly lovable roles, would he ever consider turning expectations on their head and playing a thorough-going baddie? "No," he replies. "I do recognise I have a responsibilty to the audience. They feel fairly confident that if I'm in something, they can sit down with their family and be entertained by it. I don't desperately want to change that. There again, if a super part as a psychopath came along, I might give it a whirl."
A dastardly Del Boy - now that would be something to behold.
A new series of `A Touch of Frost' begins tomorrow at 8pm on ITV
1940: Born David White (his stage name is a tribute to Jason and the Argonauts) in Edmonton, London. Son of Billingsgate porter and Welsh char. Brought up in two-up, two-down with outside lavatory in Friern Barnet, north London
1950s: After leaving Northside Secondary Modern, trained as electrician, but was passionate about amateur dramatics
1965: Big break came when his brother, the actor Arthur White, was given a role in Z Cars. To rave reviews, Jason took over from Arthur in Noel Coward's South Sea Bubble at Bromley Rep
1967: Made it into TV on comedy show, Do Not Adjust Your Set, with Terry Jones, Eric Idle and Michael Palin
1974-7: Blanco in Porridge.
1976-85: Granville in Open All Hours.
1978-81: Peter Barnes in A Sharp Intake of Breath.
1981-96: Del Boy Trotter in Only Fools and Horses.
1987: Best Actor Bafta Award for Skullion in Porterhouse Blue
1991-93: Pop Larkin in The Darling Buds of May
1992-97: DI Jack Frost in A Touch of Frost