Bono & The Edge, Union Chapel, London

The strictly acoustic Little Noise Sessions are curated by Jo Whiley, who probably has no trouble persuading some of her starry mates to appear in aid of learning disabilities charity Mencap. There's a buzz in the air this evening, then, about the identities of the mysterious "special guests" slated to kick off proceedings.

When Whiley emerges to introduce them, she asks the audience to be gentle as they're a new act and rather shy. When the two middle-aged gentlemen in question shuffle on in their civvies, they turn out to be Bono and The Edge neither noted for their shyness, nor, indeed, for their newness.

They play just four tracks, with only The Edge's acoustic guitar and piano to complement Bono's soaring vocals, but the surroundings of the Union Chapel are the perfect setting for his lyrics' religious overtones. Three classics "Stay (Faraway, So Close)", "Desire" and "Angel of Harlem" are wrapped up with a box-fresh new song, culled from The Joshua Tree sessions almost 20 years ago and now, finally, completed. It's a privilege to see one of the world's great stadium bands in such an intimate, relaxed setting.

The pair are a near-impossible act to follow, so kudos to Liam Fray of the Manchester scenesters The Courteeners, who makes a valiant attempt. He's a likeable fellow, but his songs seem slight in the context.

We Are Scientists are thoroughly engaging. Like the musical equivalent of a Wes Anderson movie, these preppy Californians share wry, whimsical banter and their songs are perfectly-framed pop treats. They sound as good in their acoustic incarnations as they do on their album With Love And Squalor.

Headliners Biffy Clyro are left exposed at the top of an intimidating bill. Puzzle, the Scottish rockers' fourth album, has been a huge hit, and deservedly so. They're probably the UK's best response to the Foo Fighters, and their strong melodies lend themselves to an acoustic rendering.

But while he may have the facial hair, singer Simon Neil is no Dave Grohl. Neil appears nervous: he whispers a few introductions, but for the most part allows his sweet songs to speak for themselves. The novelty is starting to wear thin when, from nowhere, the dour Scots trio pull out a super cover of Rihanna's "Umbrella", an unexpected end to an evening full of surprises.