It's easy to forget what a singular figure Graham Nash is. Plenty of other young musicians from the north of England blossomed in the beat boom of the early Sixties, but nobody else so dramatically changed horses in midstream.

It's easy to forget what a singular figure Graham Nash is. Plenty of other young musicians from the north of England, sparked into life by Buddy Holly, Elvis and the Everly Brothers, blossomed in the beat boom of the early Sixties, but nobody else so dramatically changed horses in midstream. Only Nash went from the clean-cut, very British kitchen-sink realism of The Hollies to the soaring folk-rock of Crosby, Stills and Nash, flourishing in two wildly diverse musical and cultural environments. Through it all, he has remained one of Britain's most undervalued songwriters.

After talking to Nash it becomes apparent that one of his most famous songs, Marrakesh Express, isn't just a breezily literal account of the illicit delights of Morocco; it's also a micro-manifesto for his entire attitude to life. "Taking unknown paths, being in strange places and expanding my awareness of the rest of the world and how other people live," as he puts it.

Marrakesh Express was Nash's welcome gift to Stephen Stills and David Crosby, the former members of, respectively, Buffalo Springfield and The Byrds who "stole" Nash to form one of the world's first supergroups. Four decades later, they've still got him. In truth, Nash was more than ready to jump ship. He had been a founding member of The Hollies - named after their hero Buddy Holly - since 1962 and had experienced great success on both sides of the Atlantic with songs like I'm Alive, Bus Stop and Carrie-Ann. Eventually, however, he felt restricted within the band, and finally left over plans to record an album of Bob Dylan covers, something he regarded as a retrograde step.

"My view of the world had outgrown The Hollies," he says. "Not as people, they remained dear friends, but as a band, yes. You see, there's a difference between people who smoke dope and people who drink alcohol. One isn't better than the other, it's just a different set of appreciations." Nash, it hardly seems worth pointing out, was very much in favour of the former mood-enhancer. More significantly, in Stills and particularly Crosby, a sublime singer and songwriter with Welsh roots, he discovered his musical soul mates, two people whose voices not only blended beautifully and bewitchingly with his own, but who also championed the new, counter-cultural direction songs like Marrakesh Express - which had been tried and rejected by The Hollies - were taking.

"I found David and Stephen really began to appreciate me as a songwriter," says Nash. "When The Hollies weren't so hot on recording my songs, it was Crosby who supported me and loved me and said, No, no, no, these are good songs, don't worry about it. It's them that's wrong'."

The leap from grey Salford to sun-dappled LA was like moving between two entirely different worlds. Nash cheerily admits he embraced everything California had to offer with "open legs, open body, open eyes, open mind, open everything. I loved my time in Laurel Canyon". It certainly sounds like it. On Reflections, last year's three-disc overview of his wide and varied career, he dedicates one of his most beautiful songs, Lady Of The Island, to "two ladies in my life", to which the listener is tempted to mutter: "Just the two, eh?"

One of the women in question was Joni Mitchell, with whom Nash had an intense and profoundly influential relationship in the early Seventies. "My relationship with Joni was a wonderful part of my life," he says. "She was a brilliant, talented woman, incredibly beautiful, very, very funny. To sit there and watch her create those masterpieces was awe-inspiring." As a songwriter, didn't it simply make you want to give up? "No, it didn't put me off. Other people may have said, Well, I'm giving up. I can't write like that, what's the point?' But we all have our voice, we all have our point of view. That's all we have, really."

Nash has clearly enjoyed himself over the years, but his immersion in the wild excesses of Los Angeles' post-hippie culture was tempered with a liberal dash of good old-fashioned common sense, a legacy of his working class upbringing in post-war Manchester. While his friends and contemporaries - including Crosby, who ended up serving jail time in the 1980s for heroin and cocaine possession - were later handed the bill for their years of indulgence, Nash somehow managed to avoid paying too high a price.

"With every human endeavour there's damage if you over indulge," he says. "That's a lesson I learned incredibly early: if you get out of your consciousness sad things can happen. I've seen it happen to many, many people and I often wonder why it didn't happen to me, but I guess I've never been an addict. For me, Laurel Canyon was like the greatest Butlins' holiday camp you'd ever seen. You'd get up in the morning having been creating music the night before, and you'd go down to Schwabs diner for breakfast and then hang out at David's house and sing songs and see beautiful naked women. In the sunshine. It was astounding."

Such days are long gone. For the past three decades Nash has lived happily in Hawaii with Susan, his wife of 32 years. He was granted American citizenship in 1978 and does a convincing impression of being a native, speaking with a syntax-mangling transatlantic twang and displaying a view of the UK that seems to have stopped evolving sometime around the Blitz. "I think the spirit of the British people is the same as ever," he says. "In the sense that they want to cling together to overcome difficulties, they have that indefatigable sense of we-can-make-it through." Later, he laughingly confesses that "I can't find my way around Manchester any more. All the places I would go are either gone or there's a freeway connecting them to a completely different place." A freeway? In Manchester?

Away from music, Nash is a commercially successful photographer and remains a dedicated activist against war and nuclear power: "It's f***ing madness to me to be utilising materials that will be deadly toxic for thousands and thousands of years," he says, "It's one of many struggles". Above all, he still sees himself still very much as a working musician. "I live in my head," he says. "I'm always creating."

And now 2009 is a special year. Crosby, Stills and Nash (still occasionally augmented with fourth member, Neil Young) are celebrating 40 years since the release of their eponymous first album, and the milestone seems to have re-energised the trio. They performed at this year's Glastonbury festival and are currently recording a new album of cover versions with uber-producer Rick Rubin. These days, however, it's the longstanding friendships as much as the music that ties them together. "I've loved David and Stephen for many, many years," says Nash. "David is a completely unique man. I've known a lot of people in music, and no one is as strange and as complicated as Crosby. And what a voice." He pauses and, finally, a hint of blunt Mancunian pokes through the American drawl. "Even though it is Welsh."


Crosby, Still and Nash play Edinburgh Castle on Saturday.