BACK in 1977, when everyone who had any musical nous was pledging their allegiance to punk and ska bands, I used to spend days on end staring at the smoke-stained walls of a shabby one-bedroom flat in Lyne Street, Edinburgh, with a few close friends. The soundtrack for those long sessions was invariably John Martyn's gentle, beguiling music. Young and naïve, we thought that any man who wrote classic dope-fuelled anthems like Solid Air, One World and Bless The Weather must be a "really, really nice guy".

So it came as a bit of a shock when a friend who vaguely knew him confided in us that John was in fact a bit of a nutter, who hung out with gangsters and wasn't averse to the odd bar-room brawl. In the years since, I kept reading similar reports of a musician whose chaotic personal life -blighted by alcoholism and drug addictions- contrasted so sharply with his mellow musical output. Eventually in 2003, I decided to try to find out more, and write a book which would answer the central question of whether John Martyn was really a peace-loving good guy or was indeed something of a bampot.

Over the past four years I've been lucky enough to meet John on several occasions. The first time was in a dive of a bar at Shawlands Cross, Glasgow, just 200 yards from the house on Tantallon Road where -as Iain David McGeachy- he grew up. It was just a couple of hours before he was due to play a sell-out gig at the city's Carling Academy but he still asked for a double vodka and a pint of cider. When I asked if he wanted anything in the vodka he looked at me with a measure of disdain and said, "Nah, that's what the cider is for!" After some refills and a 20-minute interview, he offered me a lift with his driver/friend back into the city centre. On the way, John supped on a can of a brand of extra-strong cider that's usually the preserve of the dawn patrol. I remember thinking, "This guy will be lucky if he makes it on stage, never mind performs." But later that night he earned a standing ovation from a capacity crowd.

The years of hard drinking have taken
their toll, with Martyn now weighing over
20 stone and confined to a wheelchair -
but he is still playing.

I told John I was bound to find out good and bad things about him and that -as long as they were true- those stories would all go into print. He agreed - it took me a while to realise that John revels in his notoriety, preferring to provoke a negative reaction than none at all. Gradually the book began to take shape. Along the way I spoke to fellow musicians who adore him, ex-managers who despise him and former lovers who cannot decide whether they want to throttle or hug him. I managed to track down his teenage fiancée Linda Dunning, who now lives in the United States, and who hadn't spoken to him since they split up almost 40 years ago. Reuniting John and Linda backstage at the Glasgow ABC was a really moving experience, though it's fair to say that the years have been kinder to Linda.

I wouldn't say that John took an active interest in the book: he has a natural aversion to computers, e-mails and answering letters. But he did go out of his way to persuade some of his old friends to help out - including Claire Hamill, the English singer-songwriter who had a long affair with John during the early Seventies. Claire told me that John was a "sweet, tender lover", adding -somewhat cryptically- that he always seemed to be in possession of what Claire says was "the biggest lump of dope I have ever seen in my life!" Some of those I approached weren't quite so forthcoming. One former manager told me: "You must not come to me for praiseworthy things on John Martyn. I've experienced his darker side much too often. I'm not interested in adding to the myth of the man. I don't enjoy talking about John Martyn. If you cannot say something good about somebody, don't say anything. I find it hard to say anything meritorious about John Martyn." He then put the phone down, probably unaware that he had just given me the book's introductory paragraph.

When I repeated the quote to John he seemed quite pleased, telling me: "That's wonderful! Some people are crazy about me, some people just cannot stand my face! A lot of people just do not like me. See, I always tell people if I don't like them and they are not used to that. You are supposed to love everybody - but in all honesty, I don't suffer fools gladly. I do not like any form of dishonesty. I have told lies all my life to avoid confrontations and nastiness, but nothing that would harm anybody. I think I am just a touch too outspoken and I'm rather proud of it to be honest. These people, their get well soon cards just get lost in the post. If they were my real friends, I would still be hanging with them and sending them Christmas cards and I don't. They were a transient part of my life. There's always a reason for me discarding them." So if any of you out there are in receipt of a Christmas card from John, treasure it, it means he really does like you.

Forty years in the music business he loathes, a successful battle against heroin addiction and a still unresolved struggle with alcoholism have left their mark on John. Aged 60, he now weighs well over 20 stones, has only one leg and is largely confined to a wheelchair. His musical output is less consistent than before, his concerts can be either disastrous or genuinely wonderful. He will probably never write, sing or play guitar as well as he did back in those hazy days of the Seventies, and those who love him, like his long-term musical partner and friend, the bassist Danny Thompson, fret a good deal about his lifestyle. (Only this week John's website announced that he had to cancel some concerts in Ireland due to "ill health".)

Ralph McTell, who rose to prominence with John out of the Soho folk scene of the Sixties, told me there is no doubt John should be dead by now given the abuse his body has undergone over the years. On John and Danny's partnership, McTell commented: "They made sublime music - full of madness, mayhem and unpredictability. I don't know of any of the wild men of rock'n'roll who could compare to them and the things they got up to. But, man, I wouldn't have been anywhere near them... I cannot believe how they survived such drunken and drug-fuelled strength of feeling." But survive he did and sustained by the love of a good woman and his pet dog Gizmo, John now lives a relatively peaceful life in Kilkenny.

And now the book is finished, can I tell you if John is really a good guy or bampot? Well, my experience is that he is the former - but boy I don't think I'd ever like to get on the wrong side of him.

John Neil Munro's biography of John Martyn, Some People Are Crazy, is published by Birlinn on October 11, £ 14.99 www.johnmartyn.com