British police cars in the 1950s had a bell instead of a siren. The Wolseley 6/80 used in the film clearly shows the chromium plated bell visible above the front bumper as the squad car is driven to Mrs Stokes' home. In a following scene where the same car pulls up at the chemist's shop, having been driven there directly from Mrs Stokes' house, the bell is missing.
Although it's established that the girl lives at Myrtle Mansions, the building is later referred to by police as Milstrom Mansions.
The action takes place in September 1956, judging by the calendars and the newspaper headlines. Yet the murder is referred to as having happened four years previously - which would make it 1952. But the newsstand headline of "West Indies clinch it" which appears on the photograph in question can only refer to August 16th 1950 when the West Indies clinched the last cricket test match at the Oval by beating England and winning the series. Therefore the murder took place 6 years prior to the action, and not 4 years as stated in the film.
Shadow of the camera falls on the office door of garage when Dearing opens it to find the body of Bates on the floor.