Claudio Abbado, Daniel Barenboim and Teresa Berganza are just some of the musical stars performing together at the Barbican on Sunday in a tribute concert for the late Peter Diamond. Humphrey Carpenter finds out more about this outstanding manager, promoter and director. He also blows the cobwebs off the harmonium with poet and humorist Ivor Cutler. Music includes Vivaldi and Wagner, and, at about 6.30, Walton's Cello Concerto played by Julian Lloyd Webber.