"YOU KNOW WHAT, YOU'RE OFF your chump, you are," Spike Milligan told Michael Barrymore the first time they met on a TV chat show some 16 years ago. "That's rich coming from you," responded Barrymore, then Britain's most highly paid entertainer and the reigning monarch of Saturday night TV.

"I never found out what Spike meant," says the fallen star. Perhaps he'll know what the mercurial Goon was getting at by the end of next month, though, for Barrymore is about to become Spike Milligan. He's the lead in a new play, Surviving Spike, a sort of biopic of the comic genius, a man Barrymore insists merits the title of the greatest icon of British comedy. Milligan was also a vulnerable, complex man beset by demons. Surviving Spike is Barrymore's debut on the Edinburgh Fringe, yet another comeback from the seven years of hell he's been through since a young man died in his swimming pool in March, 2001.

It sounds like dream casting: one deeply troubled star takes on the mantle of another. "I know, I know, he was either ranting and raving or down here," says Barrymore, indicating the floor. Does he identify with that? "Yeah, I do. I understand what went on in his head. When you don't feel secure, you turn to comedy and that hides the misery that's going on in your head. That's how I've often medicated myself when I've felt really crap."

There is a perception that the Bermondsey-born comedian and actor is a broken man. He isn't. As rail-thin as ever, he looks well. His eyes are bright, his cheeks pink, his skin clear, although the mop of dark hair is pewter-grey and receding. His speech often sounds slurred, but then he's never had the diction of an RSC thespian. He swears he's sober and clean now, although we shall return to this subject later.

Nonetheless, like Milligan, Barrymore's relationships have been beset by difficulties. After he came out as gay, his 18-year marriage to his wife and manager Cheryl collapsed – they divorced in 1997 and she died of cancer in 2005. He has also suffered bouts of depression.

Spike spiralled into profound depression, while Barrymore says his own biggest problem is his addictive personality, which for so long was satisfied by simply being on stage, lapping up the love and laughter of his audiences. "It was only when I got successful and I had space that I found other more harmful addictions," he says. "What was I supposed to do with the other 22 hours of the day? I've struggled with not being on stage.

"In the play, I have to lie on a cot, like the one Spike kept in his office, curled up in a foetal ball. He'd lock himself away for days and wait for the depression to pass. It's very painful for me to go through all that on stage; it's close to home."

Barrymore, now 56, interviewed Spike in the early 1990s on his chat show, Barrymore. "I was in awe of him – he was my comic hero. A brilliant, brilliant man. But it was bizarre. He sat there, slumped on the sofa opposite me – like this," says Barrymore, assuming the body language of a rag doll.

He drinks coffee and says quietly: "We've both been through a lot, me and Spike. But I don't think I'm tormented in the way that he was. He was bipolar; I'm not. If I were, I wouldn't be sitting here talking to you now. I'd be dead, after everything that's happened to me."

Barrymore is not only referring to the death of 31-year-old, father-of-two Stuart Lubbock at a party in his then Essex home, but to his problems with drink, drugs, rehab and psychotherapy. In December, the police finally told Barrymore that he will not face any charges over Lubbock's death. However, Lubbock's father continues to campaign for justice for his son.

"That's his journey – and it's the path he must take," says Barrymore, who believes that life is predetermined. "There is no such thing as good or bad luck or coincidence," he claims. "I am on my own journey and this is the latest cycle of it. Of course I've indulged in self-pity. I've asked, why me? But a close friend said to me recently, why not you? I actually felt better when he said that. However, I'm dead chuffed to be doing the Edinburgh Fringe – it's like I'm starting all over again. When I was a kid, all I wanted was to be an actor, not a comic, although I could always make my mates in our block of flats laugh when I mucked about. It was one way to laugh off my dad's drunken behaviour; it gave me a great sense of power. You make people laugh and they don't ask you questions. It stopped me being bullied. Kids can be so cruel."

At 19, he won a scholarship to Lamda, the London Academy of Music and Dramatic Art. "I did an audition as Malvolio. Oddly enough, there's talk of me playing him in a production of Twelfth Night sometime soon," he reveals. He's already done a regional tour withScrooge the Musical and is planning more stage work. He'd love it, if Surviving Spike got a West End transfer but he can't look that far ahead. He's on stage for one hour and 45 minutes in the play, so there are a lot of lines to learn.

We meet in London's Little Venice, at the offices of Bill Kenwright, who is producing Richard Harris's play. It's based on the memoirs of Spike's formidable manager for almost 40 years, Norma Farnes – played by Strictly Come Dancing winner Jill Halfpenny, late of EastEnders. Norma reminds Barrymore of his late wife Cheryl, who controlled his life and career for years, even dictating what he wore.

I have met Barrymore before. In 2005, I went to his palatial Essex bungalow mansion, where we sat on his enormous Gucci leather sofa and then stood beside the most infamous swimming pool in Britain and he gave me a revelatory interview. He was planning his first comeback: a one-man show in London – it bombed – following an appearance at the Edinburgh TV Festival, where he spoke about "Presenters Behaving Badly".

Today, he greets me like a long-lost friend, giving me a hug and immediately asking if I knew that his boyfriend of ten years, Shaun Davis, left him a year ago. "I'm on my own for the first time in my life," he says, then tells me that his Jack Russell dogs – JD and Sprite, named for his erstwhile favourite tipple – both died shortly before he returned to London from New Zealand, where he had lived for a while.

"JD died of cancer and the day before I was coming home, Shaun backed the car into the drive of our house in Auckland and ran over Sprite. I remember thinking, what the hell's going on with my life? But maybe they've left me so that I won't be burdened by them, I don't know. It's all mapped out, you know. Perhaps I'm meant to be alone on this part of my journey."

Is he lonely? "I'm managing to deal with that a lot better. I am more at peace with myself now. At first, it was weird, a bit strange. Now, I am starting to enjoy it. You know, I'd never even had to make myself a cup of tea. Everything was done for me. As I said, I feel I'm starting over. All I need is work to make me happy – it's the best high in the world."

At one point in Surviving Spike, Milligan hands Norma a gun and asks her to shoot him. Was Barrymore ever tempted to end it all? Obviously, he agrees, he was committing a slow form of suicide by drinking. Only once, one morning in early 2005, did he contemplate killing himself.

"But I stood up and had a shower and I've never felt like that since."

LAST MONTH, MICHAEL BARRYmore went out on his own for the evening in London and ended up – yet again – making red-top headlines, alongside paparazzi pictures of him apparently staggering drunk and allegations that he threw food around a West End restaurant, leaving without paying.

"I didn't even know I'd been photographed," he admits. It was, he sighs, him hitting the wall that sometimes rears up out of nowhere and with which he's so often spectacularly collided, although with nothing like the frequency that he used to. He's thoroughly ashamed of himself and vows it won't happen again.

"You can never say never, of course. However, no-one can beat me up for failing as much as I can beat myself up. I promise you, no-one is more disappointed in me than I am with myself."

After the interview, we eat a sandwich lunch together. He stands up, kisses me, envelops me in another bear hug and says: "Let's have a nice night out together in Edinburgh." Of course, I tell him, it would be a pleasure – so long as it's an evening of sobriety and laughter.

• Surviving Spike is at Assembly @ George Street, Edinburgh, from 31 July to 25 August, as part of the Fringe.