Rock meets classical, anyone? Anyone at all? Hello-o? Thirty-eight years since Pink Floyd recorded Atom Heart Mother, a 27-minute mix of pop, brass, cello and choir, this acme of prog rock retains a murky reputation. It gave the band their first No 1 album, boosted by its iconic cover shot of a cow. But even the boys themselves have since prodded at the album’s carcass with disdain. “Atom Heart Mother is a good case, I think, for being thrown into the dustbin and never listened to by anyone ever again,” the bassist Roger Waters said in the Eighties. “Absolute crap,” agreed the soft-spoken guitarist David Gilmour a decade later.

Well, a chap can change his mind, can’t he? On Sunday Gilmour was the star guest at the second of two concerts by Ron Geesin as part of this year’s Chelsea Festival. Geesin, the Scottish composer who came up with the orchestral parts of Atom Heart Mother, provided an appealingly odd first half. There were new pieces he had written for the Royal College of Music Brass Ensemble and for the vocal ensemble Canticum; Geesin on the banjo with cellist Caroline Dale; Geesin on marimba, piano and banjo and, oh yes, Geesin reading out some aphorisms. He’s a one-off, all right, even if he doesn’t half keep reminding you about it.

But it was the second half we were waiting for, and it was a revelation. Atom Heart Mother, as Geesin admitted, has its longueurs. But as Gilmour joined Mun Floyd, an Italian Pink Floyd tribute band, to play this epic alongside the classicists, it began to feel less like an affably overambitious period piece and more like a genuinely audacious and affecting piece of music. If Gilmour felt at all dubious about a track he first recorded as a 24-year-old — too young to know that such a rich mix of blues and brass and musique concrète is Not A Good Idea — it didn’t show in his playing. On form, there is no more lyrical electric guitarist alive. When he played his slide guitar in particular, it felt as if he was playing directly on the heartstrings. Behind him, the hairy boys of Mun Floyd played perfect re-creations. The brass lacked a fraction of the original’s bluster, perhaps, but Dale and the 25-strong choir helped to make a performance that exceeded its source material.

The emotional content emerged from this bold, not so say foolhardy, form like never before. The result? A performance that made this combination of elements feel natural and vital. A fabulous achievement.

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