The pips are broadcast on the radio with a long last pip. The last pip was not lengthened until 1972.
The series is set in 1952 and Susan says her husband Timothy works for the "Department of Transport". However it didn't gain that name until 1976, at the time of the series it was known as the "Ministry of Transport".
The sign at the Royal Mortuary (where our sleuths secretly obtain the autopsy records of three victims) appears to be in Benguiat typeface, published in 1977.
As Susan is knitting after the events at Bletchley, the clock starts to chime. It reads ten o'clock, but it chimes twelve times.
The departures board at St Pancras shows that a train is due to leave for Crinklewood. That should read Cricklewood.