When at Cedric Clissold's Death one of the cars seen is a Triumph 2000 mk2. This type of car was not introduced till 1969. The episode is set in 1967.
At the end when Morse and Joan Thursday are talking, a steam locomotive is heard in the background. By 1967 when this episode was set, there would no longer have been steam trains in the Oxford area - they had all been replaced by diesel trains.