The key is to follow the key! The Long Arm is directed by Charles Frend and written by Janet Green and Robert Barr. It stars Jack Hawkins, John Stratton, Dorothy Alison and Michael Brooke. Music is by Gerard Schurmann and cinematography by Gordon Dines. Detective-Superintendent Tom Halliday (Hawkins) heads up an investigation into a number of safe cracking robberies. Which in turn turns into a murder investigation. Out of Ealing Studios, this is a little cracker of a police procedural detective mystery. The flow of the investigation is natural, not given over to wild implausibilities, and…