Jason, 52, is the biggest star on British television. Anything he touches is ratings gold. Last Sunday night, 16.82 million people sat down in front of his new drama about a Yorkshire policeman called A Touch of Frost.
But for a lucky double booking 28 years ago, Jason would probably have been sitting down in front of the telly with them. The son of a Billingsgate fish-market porter and a charlady, he did not exactly come from theatrical stock. But when his older brother, Arthur, became an actor, young David was desperate to follow in his footsteps. After years in amateur dramatics, he took his chance when Arthur could not fulfil an engagement in rep and recommended his brother as a replacement. He turned professional at 25 in 1965, and because there was already an actor with the same name he changed his surname from White to Jason, using the name of his twin brother, who died soon after birth.
Jason's first success was in 1967. He was a triumph as Captain Fantastic in Do Not Adjust Your Set, a children's television show, but when his colleagues - Michael Palin, Terry Jones and Eric Idle - pursued another line, they decided not to take him with them. The working-class lad from Finchley was not, it seemed, the Monty Python sort.
He was devastated and went into a long depression, which culminated with work on Crossroads, playing a psychopathic gardener. But such was his comic touch on the soap that he was invited by the BBC to join the cast of the satirical radio show Weekending. 'He was a brilliant mimic, especially his James Callaghan,' says the show's producer, Jonathan James Moore. 'We all knew he was a great comic performer. In our radio way, we sat around waiting for telly to pinch him back.'
Jason returned to television in the late Seventies. His main ally was Ronnie Barker, who cast him first in Porridge (in which he played an ancient con) and then Open All Hours (in which he played a gormless youngster). The big jump came in 1981, when John Sullivan, the writer of Only Fools and Horses, was reluctantly persuaded that this jobbing actor was the man to tackle his south London spiv, Del Boy Trotter. Jason, third choice for the part, thought he had been called in to audition as Del Boy's grandad.
'I warmed to it immediately,' remembers Garry Bushell, television critic of the Sun, one of the programme's early champions. 'It was the modern equivalent of Steptoe or Hancock, that good. Coming from south-east London I found it absolutely specific in its truth. I thought, yeah, this is modern London.'
Sullivan's scripts were superb, but Jason's characterisation of Del Boy added the gloss. He based it on a man he knew when he was a young apprentice electrician. According to Bushell, Jason played him like a combination of Sid James and Tony Hancock, the cocky clown with a dark undertone. He transformed Del - the golden- hearted rogue, the Jack-the-Lad - into Everyman: subordinate to inferior superiors, the working-class hero.
Jason himself thought his later effort The Darling Buds of May worked because it was a piece of television that the family could watch together, without sex, violence or swearing. He has a puritanical attitude to bad language. For Only Fools and Horses John Sullivan replaced the expletives of south London with a rich invented argot. Jason revelled in it, and, in a neat example of reality reflecting art, made words such as dipstick, noofter and lovely jubbly part of a real-life wide boy's vocabulary.
With success came fame, and with that intrusion. At first reluctant to talk to the press, he entered the publicity treadmill a couple of years ago mainly to scotch the rumours about his confirmed bachelorhood. A person's sexual orientation was their own business, he said, but he wanted to make it clear he was no 'woofter'. It was just that he was too busy to marry, 'wedded to his work', he usually said.
In his youth, Jason enjoyed the promiscuous life of the rep actor. His sister-in- law banned him from bringing conquests back to his brother's house for the weekend, she was fed up with meeting women in the bathroom. His chum Malcolm Taylor, for whom he was best man, doubts whether Jason has ever given much to a woman. 'He might make them laugh,' says Taylor. 'But as for commitment, I don't think he wants to know.'
For the past 15 years he has been with Myfanwy Talog, a Welsh-language actress, with whom he shares a house near Aylesbury. He calls her his 'companion' and admits to spending less time with her than he does travelling to and from the film set.
Pushed by the actor's fear of resting ('comedy is a fickle mistress,' he is fond of telling interviewers, 'you never know who she will go off with next'), Jason works to distraction. About his only relaxation is walking his three-legged dog round the local village. He usually goes out in disguise, which, according to his neighbours, is a touch pointless when you have a three-legged dog.
Though the television establishment was slow to recognise Only Fools, when it did it was as enthusiastic as Bushell. The show won a Bafta award in 1990, nine years after its inception. More importantly, it established Jason as an actor with a rare gift: an ability to appeal across the classes, to be equally lionised by the Guardian and the Sun. It is a gift that television requires more than any other art form, because complete accessibility brings bumper ratings. Jason is a quality actor and a diamond geezer: a living combination of the Reithian values of the BBC and ITV's commercial bankability.
After taking a huge audience to Channel 4 with his 1988 Bafta-winning performance as Scullion, the grumpy gateman in Porterhouse Blue, Jason pitched up on ITV. Yorkshire TV's head of drama, Vernon Lawrence, (who has since risen to be chief scheduler of the network) wooed him with the promise of juicy comedy dramas and the serious role he craved.
First up was David Nobbs's A Bit Of A Do, which garnered the best ratings for a Friday-night drama - 13 million - that ITV had had for five years. His biggest Yorkshire success, however, came with the dramatisation of H E Bates's The Darling Buds of May in April 1991. Just after the Gulf war, with the economy moving into slump, Darling Buds offered romantic, nostalgic, optimistic escapism.
Jason played Pop Larkin, another Everyman, another jokey mover in the black economy, another survivor. The show didn't have the sharp edges of Only Fools, but it was huge. It also provided him with another, albeit irritating, national catchphrase: perfick.
For some time, it has been television producers' Holy Grail to find another Inspector Morse. A Touch of Frost, Yorkshire's new Sunday night drama, follows the Morse code - chippy warhorse detective in a provincial town with an unfortunate home life, filmed in two-hour episodes - but the icing on the cake was to cast David Jason as star. He is, in the Hollywood phrase, the best 'opener' in British television (he attracts viewers just on the strength of his name) and the programme took an audience of 18 million for its first episode.
It transpired that Jason was an accomplished technical drama actor as well as a man with comic timing. The Frost audience stayed with him. For a long time he had felt the need to find a serious role, to show himself he could do it. David Reynolds, director of entertainment at YTV, says: 'I don't think he is competitive, he does it for himself. As an actor he is so chameleon-like, the physical change from Del Boy to Jack Frost was remarkable. But it is more than just appearance, he has the ability to become the part. You believe he's Jack Frost, even though you know it's the same guy who was Del Boy as well.'
It is the same drive that keeps him working long after his bank balance has gone into multiple figures. The Comedians' Golf Society has to go about its noisy charity work with Tarbie, Brucie and Ronnie, but without Jasie. He has no time for the chat-show circuit: people who know him say this is a pity; privately he is a raconteur and a yarn-spinner, a good lad to have a beer with.
Instead of bantering jovially, this weekend Jason is finishing shooting the Only Fools Christmas special, then he will spend the holiday with his brother, sister, widowed mother and their families. In January he begins 20 weeks' work on the second series of Frost; after that there is another Darling Buds. In between are commercials (he trousered pounds 100,000 for an Abbey National advertisement) and voicing over children's cartoons (anything from Dangermouse to Victor and Hugo). Even these he approaches with a vigour bordering on obsession. A Thames Television technician, employed to record his voice for the cartoon Count Duckula, remembers him waddling up and down the studio for what seemed like hours, groping for the characterisation.
Like most British actors, Jason has made no secret of his desire to go west (his role model is Bob Hoskins) but, though he clearly has the technical ability, there must be a doubt in America whether he has the equipment for the international big screen. His characterisations have everything except, even with Del Boy, the kind of sexual charge to be big box office rather than just big ratings.
David Jason, it appears, is no Dudley Moore. And this Christmas, as we wade through British television's usual seasonal offering of dross and see his work shining out like a beacon, we should be profoundly grateful for it.