Van Der Graaf Generator, Royal Festival Hall, London

When the four core members of this almost-forgotten prog-rock band start a gig for the first time in 29 years, a joyous roar bounces round the auditorium.

When the four core members of this almost-forgotten prog-rock band start a gig for the first time in 29 years, a joyous roar bounces round the auditorium. That's how much Van der Graaf Generator mean to a small but fanatical section of their Seventies generation, and those, such as ex-Blur guitarist Graham Coxon, who have joined the cult since.

Centred around the "Hendrix of the voice", Peter Hammill, the band's jazz-influenced improvisations always retained an intensity and lyrical despair that set them apart from the likes of Yes. It is why they were spared in punk's dinosaur-cull, and praised by Johnny Rotten. And it is why this mostly middle-aged crowd is generating an atmosphere of adolescent expectation, as if this might just be a major event.

When Hammill last took to a London stage, in 2004, quite soon after a heart attack, he seemed fragile, his voice shakier than normal. Tonight, though, he throws his whole stick-thin frame into hurling words from his body, arms flapping at his side, like a marionette flung about by his own music.

It is no more than the band's signature "Refugees" deserves, as its lyrics of alienation sink in and Hugh Banton's soul organ takes flight. To some here, it's like The Beatles doing "She Loves You". But it's on the less familiar "Childlike Faith in Childhood's End", when Hammill essays an operatic baritone, that the band's bleak sense of lifelong struggle hits home.

For all Van der Graaf's improvisation, they remain in control of pace, carefully shifting volume and rhythm, even as they let each song's shape stretch, without ever snapping. Despite moments of tumbling dissonance, and the sense that any song could carry on indefinitely, there are no free-form freak-outs. The two songs from the new album, Present, grow into themselves in this environment.

But it's in the encore, "Killer", that Van der Graaf find their purpose. The sound gets insectoid and thick, the song becomes schizoid, forgetting itself, before familiar chords crash back in. And Hammill, conducting from the rear, lost in something larger than himself, seems rejuvenated. As do his transported fans.