Black and White late-60s-early 70s British murder suspense movie

Shot in b&w, set in France, British cast. Male star was big at the time. A female driver picks up a British man, who is hitchhiking (can’t remember if he had car trouble or not). He is wearing a giant sheepskin coat. Their journey together takes place down long, straight, tree lined Roman roads in France. The radio is on in the car. Every so often the news alerts people to be on the look out for a dangerous person of interest who is a maybe killer, killing at random. One assumes the girl is in danger. The girl is taking the man to her Uncle’s chateau so he can call for help. Once at the chateau it becomes clear that the tables are turned and the girl or uncle (can’t remember which) is, in fact, the killer. It’s very suspenseful. I only ever saw it once as a teenager and remember it vividly but cannot remember for the life of me, what it’s called.