Read years ago. Its in the style of Margery Allingham-atmospheric London, fog 1930/40s. Young bloke who studies and lives in spartan book-lined attic, works at night as a night watchman in a yard. Yard or factory or whatever gets burgled and young bloke is trapped in his little shed with dangerous robber who may kill him. YB persuades him to not take it any further and incur death penalty. Detective/mystery story follows on from this episode. Ideas?