Frederick Forsyth gets a (perjorative) mention in this but he enjoyed it so much that he wrote about it in his Daily Express column. He deliberately misnamed Midsomer Worthy as "Midsomer Jockstrap" and he described the whole series as "amiable tosh".
The trivia item below may give away important plot points.
This was not the only time Anna Massey played a character who attempted to mentally control and murder Joanna David's character. In 1979 they appeared together in Rebecca, with Anna Massey as Mrs. Danvers and Joanna David as the second Mrs. de Winter.