The Mirror Crack'd (1980) on Netflix

St. Mary Mead is all aflutter when movie star Marina Rudd and a film crew arrive is St. Mary Mead to film Mary Queen of Scots with Marina in the lead role. At a reception however the secretary of the village organizing committee, Heather Babcock, dies from a poisoned cocktail. The drink was Marina's who had given it to Heather after she spilled her first one. There is already considerable tension between many of the characters and as such, many possible suspects. Jason Rudd, Marina's husband and the director of the film, is having an affair with a production assistant, Ella Zielinski. Lola Brewster - Marina's hated rival and friend of the film's producer Martin Fenn - arrives having been given a small part in the film. Miss Marple and her nephew, Scotland Yard Inspector Dermot Craddock, work together to solve the puzzle.