This film is set in 1952. In the cottage at Exhampton, Miss Marple is seen speaking in an Ericofon single-piece telephone. But this phone wasn't put in production until 1954, and was adopted by the British Post Office as late as 1974.
After arriving at her nephew's cottage, Miss Marple answers a telephone call with a beige Ericofon. This model went only in production as from 1956. But the plot refers to Winston Churchill as Prime Minister, who left office in 1955.