In June 1975, while Pink Floyd were recording the album Wish You Were Here at London's Abbey Road studios, a portly, shaven-haired man arrived and stood quietly at the back, watching.

He appeared as the Floyd performed the song "Shine On You Crazy Diamond". It contains the words: "Remember when you were young, you shone like the sun. Shine on you crazy diamond. Now there's a look in your eyes, like black holes in the sky."

At first, they didn't recognise the man, whose head and eyebrows were shaved and who was apparently trying to clean his teeth by holding the brush still and jumping up and down.

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But this was the "crazy diamond" himself: Syd Barrett, the subject of the song. He was the most famous "acid casualty" of his generation, and the writer of much of the original material of the group, from which he had been ejected because of his drug-induced eccentricities.

When Roger Waters saw his old friend, he broke down.

Rick Wright, the keyboards player later told an interviewer: "I saw this guy sitting at the back of the studio... and I didn't recognise him. I said, 'Who's that guy behind you?' 'That's Syd'. And I just cracked up, I couldn't believe it... he had shaven all his hair off... I mean, his eyebrows, everything... he was jumping up and down brushing his teeth, it was awful...

"Roger [Waters] was in tears, I think I was; we were both in tears. It was very shocking... seven years of no contact and then to walk in while we're actually doing that particular track. I don't know - coincidence, karma, fate, who knows? But it was very, very, very powerful."

Pink Floyd continued as one of the biggest names in music, but for much of the time since, Barrett lived reclusively in Cambridge, painting and gardening, cycling to the shops and refusing all interviews. He preferred to be known by his original first name, Roger, and looked very different from the slim and dark-eyed genius of the Sixties.

While he had driven them to despair, Barrett was never forgotten by his former bandmates, who made sure he received all his royalties. On 2 July last year, when the Floyd, whose remaining members reformed for the Live8 concert in London, they dedicated "Wish You Were Here", to Barrett.

Just over a year later and after nearly four decades as the most famous recluse in rock'n'roll, Barrett, has died, aged 60. He had been suffering from diabetes and stomach ulcers.

Last night, the Floyd paid tribute: "The band are naturally very upset and sad to learn of Syd Barrett's death. Syd was the guiding light of the early band line-up and leaves a legacy which continues to inspire."

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Despite the fact he had not produced any original work since the early Seventies, Barrett remained an iconic, almost mythical figure in music. He was a presence whenever Pink Floyd performed, and was cited as an influence by contemporaries such as Pete Townsend and David Bowie, and groups such as The Cure, Placebo and The Libertines. In Tom Stoppard's new play Rock'n'Roll, showing in the West End, he is portrayed in the opening scene, and his life and music are a recurring theme.

Bowie, who recorded a version of "See Emily Play", the Floyd's second single, on his album Pin-Ups' said: "I can't tell you how sad I feel. Syd was a major inspiration for me. The few times I saw him perform ... during the Sixties will forever be etched in my mind. He was so charismatic and such a startlingly original songwriter. His impact on my thinking was enormous. A major regret is that I never got to know him."

Former Blur guitarist Graham Coxon cited Barrett as one of the greatest influences on his career.

Barrett was from a middle class Cambridge family and was at school with Waters and guitarist David Gilmour, although he later studied at the Camberwell School of Art in London.

Originally called The Tea Set, The Screaming Abdabs or The Megadeaths, Barrett renamed the band the Pink Floyd when he joined them in 1965. They originally played R'n'B covers.

In early 1967, they signed to EMI and released Barrett's "Arnold Layne" reaching 21 . "See Emily Play", also written by Barrett, reached sixand they followed with their first and critically acclaimed hit album, Piper at the Gates of Dawn; Barrett wrote most of the album.

Barrett soon tired of playing their hit and began to experiment on stage. The Floyd became a more improvisational group and a mainstay of the psychedelic underground music scene.

But, even as Piper was released, Barrett's drug abuse spiralled out of control, becoming a liability to the band. One night on stage he de-tuned his guitar, and on others he simply stood there staring straight ahead. In the United States, he once took the stage with a pot of Brylcreem on his head into which he had crushed a bottle of Mandrax, the sleeping tablet favoured as a recreational drug.

According to Tim Willis, Barrett's biographer, there were stories of Barrett being locked in cupboards by hangers-on and dark rumours he was being fed, without his knowledge, daily LSD doses by "friends". Gilmour would later say: "Syd didn't need encouraging. If drugs were going, he'd take them by the shovelful."

Because Barrett sometimes forgot to turn up for gigs, Gilmour was recruited as stand-in guitarist. The end came in 1968, not long after Barrett's 22nd birthday, they decided not to bother picking him up on the way to a performance. It was, said Willis, debatable, whether Barrett ever realised.

Although he would never play with them again, Barrett's career continued sporadically for some time. Once or twice, he would turn up for gigs, guitar in hand, only to be ushered away.

By that time, the Floyd were well on their way to huge commercial success, with Dark Side of the Moon, Wish You Were Here and The Wall, until tensions between Waters and Gilmour led to a final split in 1985, although Gilmour continued to lead various versions of the band until 1994.

Over the next few years, Barrett made sporadic music appearances. Assisted by Gilmour and Waters, he made two solo albums, The Madcap Laughs and Barrett, released in 1970, both revered by fans for their whimsy and madness. He appeared on Peel's radio programme, Top Gear and there was one gig in Olympia, in 1970, when Barrett walked off stage after four songs. One further attempt in 1974 to get him back into the Abbey Road studios, ended in failure.

Barrett eventually withdrew completely. He sold the rights to his solo albums back to the record company, checked into a London hotel and, after his money ran out in 1981, walked to his mother's home in Cambridge. He stayed there until his death.

No-one now doubts that what was dismissed in the Sixties as just another case of LSD abuse was more likely to have been schizophrenia, Asperger's Syndrome or another type of autism, aggravated by the drugs.

Otherwise, it was a quite life. Apart from his painting, he worked briefly as a gardener. Some of his paintings were made public, and an album of previously unreleased material came out in 1988.

And, in 2002, the BBC screened a documentary about him, which he watched at his sisters. Afterwards, Barrett was said to have enjoyed hearing "See Emily Play" again, he found much of it, "too noisy".

Shine On You Crazy Diamond

Remember when you were young

You shone like the sun.

Shine on you crazy diamond.

Now there's a look in your eyes

Like black holes in the sky.

Shine on you crazy diamond.

You were caught on the crossfire

Of childhood and stardom

Blown on the steel breeze.

Come on you target for faraway laughter

Come on you stranger, you legend, you martyr and shine.

You reached for the secret too soon,

You cried for the moon.

Shine on you crazy diamond.

Threatened by shadows at night,

And exposed in the light.

Shine on you crazy diamond.

Well you wore out your welcome

With random precision,

Rode on the steel breeze.

Come on you raver, you seer of visions,

Come on you painter, you piper, you prisoner, and shine.

Lyrics by Roger Waters