Simon Day is one of those names you recognise without quite being able to place. His face is a different matter; he’s Competitive Dad/Billy Bleach/Gideon Soames from Nineties BBC comedy The Fast Show, music hall “star” Tommy Cockles and that bloke off the Powergen adverts. He’s popped up in everything from telly’s Jonathan Creek to hit movie Shakespeare In Love.

Day, 49, is used to being better known as his characters. In fact, he prefers it that way – all the better to hide behind. He has never been comfortable baring his soul in public. Even in his earliest dalliance with fame, when he won the 1991 Time Out New Act Of The Year award, it was in the guise of Tommy Cockles.

None of which goes any way to explaining why he would choose to publish Comedy And Error, a warts-and-all memoir that looks back on the past 40-odd years of his life. “Money!” he says bluntly, when I put this to him. “You’re not supposed to say that, but to be honest, I had nothing happening at the time and a guy heard me talking at an AA meeting [Day is a recovering addict of various hues, but more of this later] and said I should write a book.”

Day, you see, has a graveyard of skeletons in his closet – thieving, homelessness, a spell in borstal and addictions to everything from fruit machines to cocaine. The man at the AA meeting wasn’t a literary agent for nothing.

At first, Day resisted – “I didn’t want people knowing I was a lying, thieving so-and-so” – but once the bank notes had been rustled a little louder, he found himself writing.

The book is a colourful read, a little heavier on regret than hilarity and a story that’s all the more surprising – though it really shouldn’t be – because of his mainstream profile. Even at the peak of The Fast Show’s success, when every school playground and office canteen rang with “Scorchio!”, “Suits you sir!” and other show catchphrases – Day was barely holding it together off-screen.

He admits it was difficult looking back: “You can’t help rehashing things and thinking of what you should have done or should have said. If I hadn’t got a career out of it, I’d probably have been a lot more bitter – and I wouldn’t have been asked to write the book in the first place, of course.”

How a middle-class boy from a decent South London family went quite so awry remains a mystery, but awry he went. The addictive personality that first manifested itself as childhood gluttony grew into an appetite for theft which eventually saw him put in borstal for a year aged 19. Things didn’t get much better on his release, when he remained on intimate terms with every pub fruit machine in the South, living on people’s floors and paying for his drink and drugs though a string of dead-end jobs.

Comedy was the thing that stopped Day completely self-destructing, that switched off what he describes as “the pilot light of rejection” that had blazed up until that point.

“I was alive at last,” he writes of storming his first gig, at the Rub-a-Dub Club in Sydenham in 1990. “I’d had an extraordinary success after a lifetime of failure and paranoia and boredom. I was the new kid in town. I was the bomb.

I was a stand-up comic.”

He found himself among kindred spirits in comedy. Maverick promoter Malcolm Hardee – “the godfather of alternative comedy” and no stranger to bad behaviour himself – became a father figure. Vic Reeves and Bob Mortimer (who he would later join on Reeves’ Big Night Out and The Smell Of Reeves And Mortimer) offered inspiration and support and his Fast Show colleagues proved enthusiastic drinking buddies, Caroline Aherne, Mark Williams and John Thompson especially.

Day notes how the show’s powerhouse Charlie Higson would often grumble: “At least Monty Python only contained one alcoholic; there’s bloody four of them in The Fast Show.”

Day found the disaster of his life thus far rich pickings for comic material – given a silly wig and a bit of exaggeration, the geezers he would run into in London pubs became characters, his days in bad jobs and nights in dodgy clubs were worked up into sketches.

He still gets a thrill out of writing new characters – in his current self-titled Radio 4 show, a series of oddballs are brought together in fictional provincial theatre The Mallard.

“I’m happier doing things I’ve written myself because you never really get offered the parts you want. I think all comic actors would like to get Timothy Spall’s career but it’s not that simple. I’m more comfortable with my own stuff because I get it and I can believe in it.”

Success didn’t rid Day of his demons of course and he wishes now he’d made more of opportunities instead of spending so much time “recovering from benders, sitting round watching telly and eating crisps”. He has been sober (mainly) for a few years now – having children limits one’s capacity for hangovers – but the urge to take things to excess never really goes away. “Even if I have ice-cream, I’ll just keep on eating it. I certainly have to watch myself.”

He doesn’t think it mere coincidence so many performers struggle with self-destructive tendencies. “If people are happy they don’t step on to a stage – it’s fairly obvious that people who want to be other people aren’t quite fulfilled in themselves.”

He hopes his two young children don’t go anywhere near a spotlight when they grow up. “People sometimes say my son’s a chip off the old block and wonder if he’ll become a comedian – I hope not. I wouldn’t want him to go through what I’ve been through. I want him to be a lawyer for Amnesty International or something.”

But he’s proud of writing Comedy And Error – especially given the dyslexia he has struggled with since childhood – and hopes it will offer succour to those who have lost their way and a few laughs for the rest. There’s even a happy ending of sorts; Day thinks he’s at his best now, with a family and a steady career and sobriety. He appreciates it could all have been very different. I wonder where he thinks he’d be if he hadn’t found his calling in comedy. Tellingly, he has no idea.

l Comedy And Error is out now, published by Simon and Schuester and priced £18.99 l Day will talk more about his book at the Ropetackle Centre in Shoreham on Thursday.

For tickets call 01273 464440.