Despite a few liberties with a tight original novel, this is a well realised romp balancing the potential horror of a growing mound of cadavers with wit and aplomb. It's rather sad to note one had to wait thirty years for the Brabourne & Goodwin productions of the 1970s for decent screen adaptations of Christie's cinematically oven-ready novels. The intervening adaptations were largely misjudged, flat or dire.
Up to 90 minutes in: wryly staged and deceptively genial, delivered with an assured lightness of touch. After 90 mins: you've changed the ending and ruined everything! See comments for spoilers that explain why... !
(Still leagues ahead of the soulless 2015 BBC adaptation in wit, charm and sophistication, even in spite of the changed ending.)
The wimpy excuse for Vera's character upsets me the most. She was the most interesting part of the novel, yet here her past and personhood are completely erased. The ending was adapted from a different play by Agatha Christie, so at least it wasn't totally bogus, but I still wish that this movie had the same ending as the book.