The detectives are told that Margeaux had family in 'Cumbria'. The county of Cumbria did not come into existence until 1974, being an amalgamation of Cunberland and Westmoreland.
The series supposedly taken place in 1972 shows the inspector looking at a name on a matchbook cover about one third through the show. The matchbook has it striker on the back. Strikers didn't change from the front to the back until 1973 or thereafter.