"I've got Parkinson's disease," Sir Billy Connolly writes in his new book, Tall Tales And Wee Stories, "and I wish he'd f***ing kept it to himself!"

When I meet him in a hotel suite in London's plush Mayfair for this interview, Connolly is having one of his better days. White hair cascading to the collar of his denim jacket, a chunky metallic ring on his right hand, he navigates the room cautiously but delivers a firm handshake. He looks older - he is 76 now - but the only obvious sign of his condition is the constant quivering of his left thumb.

"I'm OK today," he says. "I have bad days when I shake a lot and I can't put my money in my wallet. I know it's going to get worse, but I'll take it as it comes."

Earlier this year, the Glasgow-born comic announced his retirement from touring. But what the stage has lost, the page has gained.

A life told in stories

When the publishers first approached him, Connolly, also known as 'The Big Yin', had never written a book. In fact, he hadn't written much at all. "Most comedians write their stuff and then go and do it," he says. "I do it and then I write it down." He walks on stage with a list of headlines ("parachutists, alcohol, scrotum"), and the comedy makes itself known along the way.

In print, he's more disciplined - but thankfully not much. Somewhere between a memoir and a greatest hits album, the book weaves its way through autobiographical anecdotes, idiosyncratic observations and highlights from historic routines, often leaving it to the reader to dissect fact from fiction.

It's a fitting format for Connolly - always more storyteller than quickfire quip machine. "I think most comedians are now," he offers. "People used to come on and tell Irish jokes and mother-in-law jokes. It's all changed now, and I love the way comedy is going - the level is so high."

His comedy broke down barriers - consider the side-splittingly vulgar 'bike joke' he told on Parkinson in 1975 - but Connolly never set out to be avant-garde. "I do what I find funny," he says, "and if I find it funny somebody else will. I'm not deliberately provocative."

Instead of good taste, Connolly takes aim at 'beigists' - the cardigan-wearing, memo-wielding mobsters turning the world into "one massive bore", decrying everything from pop station DJs to obnoxiously loud sirens.

He's particularly steadfast in his defence of profanity. "It's in the rhythm of how we speak, and the colour of how we communicate," he reasons. "So if you're likely to be offended by swearing, you can..." (well, you can guess the rest!).

Complex beginnings

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As long-term Yin-watchers may know, Connolly didn't start out in comedy at all. His stage debut came as banjo player and folk singer in a duo with rock A-lister Gerry Rafferty, with whom he performed charmingly innocuous ditties about cuckoos and chicken pie. "You had to introduce your songs, and I used to introduce them funny, in a light, sometimes abstract way. That got the shape of my comedy going."

There's a strand of modern culture that pines for the 1960s and Seventies, and another that sees this as sentimental, rose-tinted twaddle. Connolly is in no doubt which side of the fence he falls.

"I look back on it with great joy. It was a great bunch of people - folkies, jazzers, poets, storytellers, people that knew obscure music and played weird instruments, all of them vaguely hippy and strange. It was totally unique."

If there's one word that describes Connolly's rise to stardom, it is 'organic'. He never really choreographed his comedy, nor calculated his career moves, and he certainly never coveted fame.

His humour connected with people because it was drawn from them. "I've always admired ordinary people - the electrician, the nurse, the secretary. You'll see them in the pub, roaring with laughter, not a comedian among them," he says. "Ordinary people are great at comedy. They wish they could do it - they don't know they can."

He learned from the best - the "patter merchants" at the Glasgow shipyards, where Connolly earned his first wage. Every tea break, the men gathered round and told tales - rough, rude and hilarious. "They were heroes," says Connolly. "They didn't change anything, but they changed me."

His baptism came years earlier. A natural class clown, he fell into a puddle aged seven, and was so gratified by the other kids' laughter that he decided to stay in a little longer. "When you're vulnerable, you're funny," he says, "and most comedians have a darkness in their lives. You turn it to suit yourself - you've survived it and it gives you this comedy edge."

Connolly has darkness to spare. He was constantly belittled by his aunts, who resented having to raise him, regularly beaten by his schoolmasters, and sexually abused by his father.

"You can get rid of most of it," he says, "but there's a bit that will always stay with you. I hate watching afternoon television, when you see a woman on with her child who's been sexually abused, and she'll say 'her life is ruined', with this wee girl just sitting there.

"You have to be careful how you treat people, because they'll come to ways to treat it themselves. I found it with forgiveness - once you forgive a person it floats away from you. You're the victim - you should be free."

Then and now, optimism is Connolly's default state. "I was always a reader and a dreamer," he says, "and had a great expectancy for the future. My dodgy past has affected everything I said or did, but it's never depressed me."

A time to reflect

No longer performing, and aware that his stage days are behind him, Connolly has had time to reflect on a career that will live long in many people's memories.

He's reticent to single out highlights - "there's lots of brilliant times!" - but that first Parkinson appearance undoubtedly sent his stock into the stratosphere. "I signed an autograph for a Chinese guy at Heathrow," Connolly remembers. "And when I got back to Glasgow, the whole airport foyer applauded."

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Fame still blindsides him - "you don't wake up famous, you wake up scratching yourself like everybody else" - and he's never been one to chase accolades. Twice topping Channel 4's 100 Greatest Stand-Ups, Connolly found the label simply added pressure, like carrying around a heavy rucksack.

More to his taste is the limitless affection of his home city of Glasgow. "I've got a funny fame in Glasgow," he says, beaming. "I'm now a relative of yours. People slap me on the back, and pull my hat over my eyes. It's just friendship - they know me now, and I know them."

He was diagnosed with Parkinson's disease in 2013, and insists it's been "dead easy" to laugh about. He integrated the illness into his act, calling out the "symptom spotters", and entering the stage to Jerry Lee Lewis's Whole Lotta Shakin' Goin' On. For performing, the curtain may now have fallen, and Connolly is candid about his worsening condition.

"It's like living with another guy, it's weird. My wife sleeps away from me because I punch her in the night. I recently went fishing in Utah with my son, and he said I was laughing and singing in my sleep. But the next night, I was fighting again - 'Argh, ya bastard.' I have to choose where I sleep and who I sleep with very carefully."

He breaks suddenly into a hearty laugh. "That", he says, eyes twinkling, "is very good advice!"

Tall Tales And Wee Stories by Billy Connolly is published by John Murray Press, priced £20. Available now.