Just Ask for Diamond (1988)

Thirteen-year-old Nick and his slightly dense older brother Herbert run the Diamond Private Detective Agency above Camden Town Tube Station in north-central London. When a master criminal called The Falcon dies, they come into possession of his box of chocolate Maltesers, which contains the secret key to a fabulous cache of diamonds. Can they unravel the mystery and avoid the clutches of seedy lowlifes Brenda Von Falkenberg, Gott and Himmell, The Fat Man and the dogmatic Chief Inspector Snape, all of whom want to find the swag first.