Mark Lawson discusses the songs and shows of Stephen Sondheim, who celebrated his 80th birthday earlier this year, and whose work is performed in a BBC Prom this Saturday. With guests including Maria Friedman, Simon Russell Beale, Gareth Valentine and Stephen Sondheim himself. Saturday's Prom is broadcast on Radio 3 and BBC2. Into the Woods is at the Regent's Park Open Air Theatre from 6 August - 11 September Producer Timothy Prosser.