Gently uses a Maglite flashlight to illuminate the tank in which the body is found. The series is set in 1970 but the MagLite was not introduced until 1979.
Anachronism on George Gently,(Gently Liberated) Rachel is interviewing Janet Ellery. On the table is a Tupperware child's sippy tumbler with footed bottom in orange. This was not available in the 60s/early 70s.