"An unstable entity" is how drummer Guy Evans describes Van Der Graaf Generator. And he should know. It's a state the group manifested as far back as mid-1969, when they split up before recording their debut album The Aerosol Grey Machine (1969), after all their gear had been stolen from outside London's Speakeasy club. But, luckily the story didn't end before it had begun.

The first album was promising but tentative; at this time Van Der Graaf Generator were still searching for a signature sound. After a reshuffle the line-up now comprised Evans, Peter Hammill on vocals, guitar and keyboards, Hugh Banton on organ, David Jackson on saxophones and flute, and Nic Potter on bass. By the time the group recorded The Least We Can Do Is Wave To Each Other (1970), for Charisma Records, they sounded increasingly confident, their music developing a distinctive metallic sheen through the combination of organ and electric sax. Hammill began pushing himself further out in both his singing and in his writing, penning tracks as diverse as "Refugees", an achingly sad document of friends drifting away, and "White Hammer", set in the time of the Spanish Inquisition. Some of the group's tunes landed around the ten minute mark, but they have an immediacy, a purpose about them. Van Der Graaf Generator resisted the instrumental waffle and grandstanding solos peddled by some of their flashier peers in the nascent progressive rock scene, preferring more direct, bolder statements.

"There was this peculiar circuit of clubs all over the country," explains Evans, "really very out-there places where you would need to make a very fundamental connection with people for it to work at all. That was good for us and I think it was the foundation."

Van Der Graaf Generator took their music still further out on H To He Who Am the Only One (1970). The departing Potter only guests on the album, leaving Banton to play bass lines on the organ bass pedals, which added another unusual timbre to their sound. The group's increasingly volatile chemistry was partly down to the musical tastes of its members: a drummer and saxophonist who were hip to the ecstatic free jazz of John Coltrane; a former church organist who'd been tutored at Wakefield Cathedral and was enamored of Jimi Hendrix; a vocalist who had been a teenage mod and soul boy, and whose own erratic, lopsided musical figures were inspired by John Lee Hooker's slip-sliding sense of rhythm.

"We had something of a jazz attitude," says Hammill. "We were interested in the fundamental chaos that is exciting about jazz, is exciting about blues and soul, and is exciting about rock music."

All this coalesced in an unprecedented way on the notorious "Killer", a song of alienation, loneliness and self-destruction. Do the auditory equivalent of squinting and those verse lines, if buoyed up by a horn section, were something that Otis Redding might have sung over (although admittedly Hammill was singing about death in the sea, rather than the dock of the bay). In the instrumental section their jazz attitude manifests itself in everybody playing across each other in a near free-for-all. Even after repeated listenings, it always feels like it's going to fall apart. Banton's Hammond C3 organ fed through a Fluid Sound Box and customized flanger comes in, sounding like the last bellows of a beached sea monster. Following that, Jackson's sax solo is an anguished exercise in pure skronk.

Playing live they were the musical equivalent of extreme sports. Van Der Graaf Generator played without a net and occasionally fell, spectacularly. But this was taken as an inevitable part of the process in which, lyrically and musically, they were always reaching for something, just out of their grasp. And if, in trying to achieve this transcendence, these four exceptional musicians risked falling flat on their faces, then so be it. After all sometimes sacrifices have to be made. When it worked, as it mostly did, it was like nothing else.

Van Der Graaf Generator's approach was the antithesis of those contemporaries who had become part of show business with all the trimmings: stage props, costumes, flash pods and extravagant lightshows. Hammill explains the group's position: "We were an underground band, before prog. A major point of departure, very early on, was that part of the prog schtick was you were supposed to present your music consistently from one show to the next. We diverged at that stage, because we were absolutely not interested in playing the same tunes on consecutive nights, let alone playing the same tunes in the same way."

Progressive rock was becoming big business, though, and the musicians in Van Der Graaf Generator certainly had the instrumental chops to mix it with the best. Hammill, meanwhile, had quickly matured into one of the most distinctive songwriters and vocalists of the era, and he was good-looking, as well. In theory, the prize was there for the taking, but the group had a different agenda: they went off to record one of the most extreme albums of all time.

Pawn Hearts (1971) found Van Der Graaf Generator taking all the pastoral visions, the gentle, whimsical eccentricity that had run through English psychedelia in the late 60s, and souring those visions horribly. The disquieting infra-red-style inside cover shot depicts the group enacting some nameless ritual in the grounds of an imposing house; the garden looks diseased, the musicians pallid and irradiated. The musical landscape sounds exactly like the cover photo looks: psychedelia's mind expansion nightmarishly turned in on itself.

Fantastically thrilling, Pawn Hearts encompasses towering structures, chaos, and extremes of beauty and ugliness. Hammill sounds increasingly unhinged, hysterically declaiming lyrics that deal with dystopia, insanity and existential struggle. Preposterous though Pawn Hearts undoubtedly is, it's also an utterly compelling sonic document of a group wrestling with their instruments, with each other, with the material and with themselves. The group toured, worked on some more material then split up. It would be tempting to say that Pawn Hearts was a sort of artistic black hole that sucked in and destroyed its creators. The band then split in August 1972, but would return…