An interesting snippet of information from IMDB:
This is the first episode of "Endeavour" to feature (albeit briefly) a real-life person depicted by a member of the cast. When Dorothea Frazil, played by Abigail Thaw as always, attends the Women's Liberation Movement conference, she is introduced to and shakes hands with the organizer Sally Alexander, who is referred to by name. Ms. Alexander is seen only for a brief medium close-up and has no dialogue. Just as the real Sally Alexander was Abigail Thaw's mother, she is played (uncredited) by Abigail Thaw's daughter, Mollie-Mae.
That means that Mollie-Mae was playing her own real-life grandmother.