On the Fiddle (1961)

Tricked into joining the RAF by a wily judge, wide boy Horace Pope sets his sights on the main chance, teams with slow-witted, good-hearted gypsy Pedlar Pascoe, and works up a lucrative racket in conning both his colleagues and the RAF. By means of various devious schemes Pope and Pascoe manage to avoid the front lines until they are sent to France - where they find themselves making unexpected and uncomfortably close contact with the enemy.