Murder at the Gallop (1963)

The old and wealthy Mr. Enderby (Finlay Currie) dies of a heart attack but the ever suspicious Miss Jane Marple (Dame Margaret Rutherford) has her doubts. Who or what gave him a heart attack? Enderby's poor relatives gather at the The Gallop, a combined boarding-house and riding school. Miss Jane Marple also gets there to find out if any of them had any particular reasons to see him dead.