Only Floyd could start out with a vaudeville style song that decsends into a full on Canned Heat boogie with a Trombone thrown in for good measure! Yet it still sounds like Floyd, that's why they are so legendary.
This was an unreleased Waters composition, only ever played live as Rest in The Man-The Journey suite. It also featured Roger Waters on the trumpet. Biding My Time was the original working title of the Roger Waters solo album Amused to Death.
Of course it's Nick. He just seems to be drumming with a completely different dynamic. This early in the band's career, or at any point for that matter, there would be no explanation for Nick Mason not being on a recording.