Exactly what kind of place John Martyn occupies in today's persistently genre-defined music scene is difficult to say. His continually evolving odyssey of a career has seen him experiment with influences ranging from folk to blues, soul, rock, reggae and funk. Some even call him the godfather of trip-hop. Thrown into the mix is the fact that Martyn is something of a musical Jekyll and Hyde, a virtuoso within whom the hellraiser and the hopeless romantic are continually at war. Oh, and he's 60, weighs 20 stone and only has one leg. He probably shouldn't be, but he's still alive. And he's coming to Cambridge.

When he picks up the phone, it is the Surrey/Cockney Martyn who greets me, rather than his semi-mythical Glaswegian alter ego which, legend has it, can only be invoked by copious quantities of alcohol. He is curiously reticent about his whereabouts of the previous evening: "I just wasn't around", which, when you hear that a famous hellraiser has gone AWOL, seems a somewhat unsatisfactory explanation. Born in Surrey as Iain David McGeachy, the only son of two opera singers, Martyn's parents separated when he was five and Martyn was brought up in the rough-and-tumble Southside of Glasgow. He started playing the guitar in his teens, having been tutored by the famous Indo-Scottish folk troubadour and raconteur Hamish Imlach. The eccentric Imlach is largely responsible for Martyn's famously funny onstage banter: "He taught me how to deal with the world of entrepreneurs and hustlers and getting ripped-off and things...how to heckle the audiences and all that kind of stuff. He taught me all the hard things. He's a lovely man." Martyn moved to London and his first album, London Conversation, was released in 1967, when he was 20. Very much a folk-based, entirely acoustic album, featuring intricate, crystalline guitar fingerpicking, rumour has it that before London Conversation was recorded Martyn had only been playing the guitar for a few months. For such a recent beginner, his playing on the record is sickeningly good. "I'd been playing eleven months," he admits modestly. "But I didn't do anything else, you know. I sat and played and played and played and played...went out with a few girls and came back and played and played and played and played." Many of Martyn's first musical offerings were influenced in a large part by the work of legendary folk and blues guitarist, Davy Graham, an often-overlooked influence on the current nu-folk and psych-folk movement. Martyn heaves a sigh. "It's almost criminal, dear boy. He's not to be overlooked at all. He's actually the best - I mean, without him there would be nothing." Fortunately, Martyn himself does not seem to have been overlooked by younger music fans, and Martyn's unique style of guitar-playing continues to influence a new wave of young guitarists. "I've listened to a lot of other chaps - you know, really, really young people and they all play with the backslap technique which I invented. I hear myself in a lot of places, and I take it as a great compliment." Martyn's famous ‘backslap' technique is difficult to describe but basically consists of his using his thumb and forefingers to deaden the still-resonating strings in between fingerpicking in order to provide a kind of percussive punctuation to solo acoustic pieces. Listen to ‘May You Never' and you'll get the general idea.

When I speak to him, John Martyn is at home in Kilkenny, Ireland, about to set off on tour. But this is no ordinary tour. Martyn is revisiting his 1980 album, the emotionally-fraught and intensely personal Grace and Danger, an album that Island producer Chris Blackwell thought too disturbing to release. Fans and critics alike have described Martyn as being better than ever on the current tour, but why Grace and Danger? "The only reason I'm doing it is because people asked me to" he says insistently. "I was half in denial, because I thought it would be too miserable, and I forced myself to listen to it and it's not at all - in fact I think its downright cheeky. There's no wrist-slashing involved. It's just an album about divorce." Martyn wrote the album subsequent to his divorce from fellow folk-singer Beverley Martyn, whom he had married in 1969. Certainly, Martyn's method of writing (though his songs are not all as intense as what you'll find on Grace and Danger) relies greatly on the spontaneous communication of emotion, to the extent that Martyn has become famous for writing songs in the morning and recording them in the afternoon. "I prefer it that way, yeah. Unfortunately you can't force it, the damn thing just happens by itself. You can try to force it, but it doesn't really work that way." Having said that, the emotional sources of some of Martyn's most famous songs can be easily traced. ‘Solid Air', for example, is a song written for and about Martyn's close friend, the legendary singer-songwriter (and former Cambridge undergraduate) Nick Drake, who died suddenly of an overdose of anti-depressants in 1974. Martyn is naturally reticent on the subject. "It's become a bit more about more than one person. It's a bit wider than that, but in the essential kind of thing about it, it's cool." The autobiographical sources behind the songs range from the tragic to the absurd. ‘Big Muff' traces its origins to a humorous incident involving Martyn, teapots and Jamaican reggae legend Lee ‘Scratch' Perry (too long to narrate here in full, but when you listen to the song it makes sense). Does he still write songs based on particular incidents? "No. Mostly it's The Gospel According to Johnny Boy, but I have the odd sort of off-the-wall moment. There's always something obscure..."

John Martyn is 60 - can we expect to see more of him in the future? "Absolutely. I shall die in harness, dear boy - with a large spliff and a bottle of champagne!"