Anthony Powell's 12-volume epic, 'A Dance to the Music of Time', is widely regarded as the most formidable single work of British fiction since the war. It is also largely entertaining: its cast of 400 characters ranges from upper-class drawing-rooms to Bohemian London, and a violent death in a hippie commune. They have, in their turn, gathered a devoted set of fans among English-speaking readers. Tonight's portrait of Powell includes tributes from such admirers as Clive James, Kingsley Amis, Alison Lurie, Robert Conquest, and Hilary Spurling.