This is a sweet little English mystery. It has some potential, but no surprises. I quite liked the actors.
Overall, the movie feels like it would be part of a series, yet not the FIRST one. I was puzzled by the many references to backstory not explained: antagonistic history with the psychiatrist, mentions of Miss Willoughby being a bit a sleuth, lack of details about the books she authored, the detective's reference to trouble often finding her. Initially, I thought there must have been a previous movie, but that doesn't seem to be the case.
The lack of basic logic is hard to swallow. With a woman having hallucination and paranoia, who is also on medication, it seems like the first step would be to verify that the medication is what it should be. I'm also curious how such a well put together woman manages without a purse. She's wearing skin tight riding breaches. I suppose there are pockets in her jacket, although it's fairly tailored. Are the pockets large enough to hold keys, cell phone, hairbrush, extra makeup....the things we women usually carry around?