Strong Poison

Strong Poison is the title of a novel (1931) by Dorothy L. Sayers and the movie based on it. In this one Lord Peter Wimsey (played by Edward Petherbridge[?] in the film) meets Harriet Vane, an author of detective fiction. The immediate problem is that she is on trial for her life, charged with murdering her former lover -- if Lord Peter does not prove she is innocent, he will lose her before he even persuades her to accept his proposal of marriage. But all the clues point to Harriet as the one who gave Philip Boyes the arsenic that killed him.