Valentine's early stage credits include: the premiere of Arnold Wesker's Chicken Soup with Barley (Royal Court Theatre, 1958); John Osborne's Epitaph for George Dillon (Royal Court Theatre, 1958); Australian drama The Shifting Heart (Duke of York's Theatre, 1959, with Leo McKern); John Mortimer's Two Stars for Comfort (Garrick Theatre, 1962, with Trevor Howard); the original cast of Half a Sixpence (Cambridge Theatre, 1963, with Tommy Steele); and The Platinum Cat (Wyndham's Theatre, London, 1965, with Kenneth Williams).