11am: U2 and Paul McCartney have just finished a rocking sound check in Hyde Park.

The stage was packed with people and there were a few hundred on the main field (even though the audience have not yet been allowed on site) as Paul McCartney joined U2 to rehearse the Beatles classic Sgt Peppers' Lonely Hearts Club Band.

With McCartney joining Edge on guitar and Bono joining him on vocals, it sounded pretty great! (U2.Com, by the way, had snuck on stage too...)

It's no secret that this is the track that will open Live 8 this afternoon ­ the original Live Aid concert for Africa was twenty years ago - and the five musicians went through it three times to get it right. (U2 were also rehearsing until late last night elsewhere in London).

'The Macca...Paul McCartney!' announced Bono at the end, to widespread cheering.

'U2!' responded McCartney in kind.

'A song written by Britney Spears!' joked Bono as The Beatle left stage right where he remained to watch U2 rehearse three more songs.

It all seemed pretty relaxed ­ in fact the whole vibe here this morning is relaxed. Amazing, given that this is the biggest rock'n'roll show in history and even, we're told, will have the biggest televised audience for any event ever!

More soon.