Lovely jubbly, John

This weekend John Sullivan will put his feet up, open a good bottle of claret, and get down to re-reading David Copperfield. In between chapters he will no doubt indulge his other pastime, apart from good books and fine wines: worrying.

Sullivan has plenty to celebrate this weekend, with the news that the final episode of his long-running sitcom, Only Fools and Horses, broke all records for British television viewing figures. But, like Del and Rodney Trotter, his slightly shady heroes, Sullivan is acutely aware of the fragility of fortune.

His taste for Dickens, to whom he returns often for inspiration, he credits to his one-eyed English teacher, Mr Trowers, a frustrated thespian who used to act out David Copperfield and Oliver Twist in front of his bemused pupils at Telfer Scott secondary school in Balham, south London. Thirty- five years on Sullivan has, like his literary hero, achieved enormous commercial success through mastery of eccentric characters, sharp dialogue and plots in low-life London settings.

Figures announced last week showed that the Fools episode on 29 December was watched by more viewers than anything in the history of British television, making Del and Rodney more popular than Dirty Den, Bet Gilroy, Gazza or Princess Di. A total of 24.35 million viewers saw the hour-long special.

Yet despite being Britain's most successful television comedy scriptwriter - other credits include the sitcoms Citizen Smith, Just Good Friends, Dear John, Sitting Pretty and last year's wartime drama Over Here - and winning clutches of awards, Sullivan is still insecure about his output.

This week he will be back at work on commissions for two new comedy series. Even if the jokes dried up tomorrow, he has already earned so much that he need never work again. So what does he have to worry about? The answer lies in his background, not unlike Del's and Rodney's, and his extraordinarily long struggle for success.

Gareth Gwenlan, Fools producer since 1987 and former BBC head of comedy, says: "Despite his success, John is always afraid that his next script will be his last and that one day he won't be able to do it. Under the skin he agonises over whether his work is any good."

Tony Dow, the director of Fools, asked Sullivan to be best man at his wedding five years ago. "He wrote a hilarious speech," says Dow, "but he was so terrified of speaking in front of all those people that he couldn't deliver it. He only managed to get through about 30 seconds before he dried up. At his 50th birthday party, just before Christmas, everyone was looking forward to a cracking speech, but again he wouldn't do it."

John Sullivan was born on 23 December 1946, in Balham. His parents, now dead, worked respectively as plumber and char lady. He has one older sister. The family was poor and life, says Sullivan unsentimentally, was hard. After leaving school at 15 with no qualifications and no idea about what he would do with his life he realised he "knew nothing" and became an "evening class freak", going to German and English classes and buying a 2s 6d (12.5p) "Teach Yourself" book in a different subject every week.

His first job was as a messenger at the Reuters news agency, followed by a period in the second-hand car trade. "I was 17, earning good money and enjoying myself," he has said. "There were a lot of villains, quite a rich seam to tap into later when I started writing. I never got involved in 'deals', though. I was always too frightened. There were a lot of opportunities if you wanted to go that way, but I didn't want to bring any problems on my family. I was never tempted to take chances."

In 1966, aged 19, Sullivan was stacking crates in a brewery for a living with an old schoolfriend, Paul Saunders. Sullivan says: "One day Paul read about the kind of money Johnny Speight, who wrote Till Death Us Do Part, was making and he said to me, 'why don't we have a go at writing a script?' We wrote this thing called 'Gentlemen' about an old war veteran who was the attendant in one of the old-fashioned toilets with all the brass pipes and tiles; it was his pride and joy. The council opened another, more modern toilet down the road, complete with piped music and aftershave on tap and he started to lose his customers.

"The script was duly rejected by the BBC. Paul lost interest in writing, but I loved the experience. I'd enjoyed creating the characters so much that I carried on. I'd never experienced that buzz before and I knew it was going to be my life."

Over the next 10 years Sullivan moved from low-paid job to low-paid job, including window-cleaning and laying carpets at the House of Commons, spending his evenings at the typewriter and piling up rejection slips. "I learned a lot by watching every TV comedy I could, good and bad, and trying to figure out why they did or didn't work."

Sullivan admired Steptoe and Son, Till Death us do Part and Phil Silvers' US show, Bilko, and "anything by Neil Simon".

Then he made his smartest move: he got a job as a scene-shifter at the BBC TV comedy department. The job entitled him to drink in the BBC Club, where one night he plucked up the courage to talk to Dennis Main Wilson, the producer of Till Death us do Part, and tell him about this idea he had for a comedy about a Tooting revolutionary called Citizen Smith.

"If that was rejected I was going to give up, but Dennis said: 'Just go and do it.' I took two weeks off to write it and he got the pilot on the screen in eight weeks."

Citizen Smith, starring Robert Lindsay, built Sullivan's reputation as a scriptwriter and Just Good Friends, the love-hate romance series, cemented it. But Only Fools and Horses, which has steadily gathered popularity since an indifferent reception on its launch in 1981, has taken him to scriptwriting superstardom.

Why should a couple of dodgy dealers who live in a Peckham tower block called Nelson Mandela House and drive around in a battered Robin Rialto van have captured the nation's hearts?

Sullivan says: "There's a lot of warmth in these characters and their community. They may take each other for a couple of quid here and there, but there's a lot of loyalty and reliability in them. A woman once said to me 'I'd love to marry a man like Del'. She understood that he may be a scallywag, but beneath the flashy exterior he's loyal and he knows how to love.

"Whatever happens to the Trotters, the family stays intact. There's something solid about them. Maybe people respond to that. And Del never gives up his dreams. People like his perseverance."

The fact that the series is brilliantly funny may also have something to do with it. Sullivan's current projects, which should come to our screens in the autumn, are Roger, Roger, a comedy drama about a minicab firm, and Heartburn Hotel, a sitcom set in a Birmingham hotel.

Tony Dow says: "He's a perfectionist, but not a prima donna. His scripts are never on time and usually half-an-hour too long and you end up cutting jokes rather than the plot, because that's uncuttable - it all fits together so well. Most writers would be happy with one good plot line in an episode, but John has three or four - and they all work out beautifully.

He's a real comedy writer, in that he makes everyone laugh. He doesn't just appeal to 65-year-olds who want something cosy, or to students who want something clever-clever, but to all ages and classes.''

Gareth Gwenlan says: "John laughs at something Del Boy says as if it's a surprise, as if he hadn't written it and it's really Del saying it. I find that hugely endearing. He is mesmerised by his creative ability. It's still exciting to him after all this time."

John Sullivan now lives not far from Balham in a large house near Reigate, Surrey, with his wife of 23 years, Sharon. They have three children: Dan, 21, James, 18, and Amy, 13. Summer holidays are a rented villa in Spain or a house in the US. Dow says: "He's not an adventurer. If you suggested going on something like a safari he'd react in horror. His pleasures are good food and wine and spending time with his family." Politically, says Sullivan, "I was brought up a socialist, and I still am."

He met his old English teacher, Mr Trowers, in the street back in the 1970s, around the time he was starting to make it as a writer, and thanked him. Mr Trowers said he had given up teaching and was working as a dustman. Del Boy would appreciate the irony.