This very poor rendering of Christie's novel is the result of putting together a team of people who don't trust their source material. So, it has to be improved. We get a new but superfluous sub-plot alluding to WWII, just so that two extra characters can do mysterious things and may have a motive for killing the nasty colonel. The poor poacher is packed off to prison at the start so they don't have to really include him. Miss Marple is the victim of a spurious accident and spends most of the film hobbling around (quite rapidly) with a stick, and has too much screen time. The policeman is amazingly friendly, discussing suspects, motive and opportunity with several villagers simultaneously. St Mary Mead isn't just an ordinary English village, it is amazingly pretty with lovely houses often photographed in the setting sun. Except it isn't real - because if you study the editing you can see it isn't a single place but an illusion constructed with jump cuts.
Geraldine McEwan plays Marple reliably enough but with too much of a jaunty, almost saucy air. And why not, when a manufactured and immoral past is inserted for her just so she can sympathise with another character in an unlikely way? Stephen Tompkinson is the dim and incompetent policeman. Meanwhile Janet McTeer is an unhappy wife with a secret, who wanders around the village wearing a succession of wholly unsuitable dresses that no-one in her position would wear at such times and places. Of course she looks very well in them, as does Christine Coleman as Lettice, who shouts at her father and models rather fetching 1950s leisurewear while lusting after the romantic artist. Even the vicar's wife, who is meant to be just scatty and very kind, is also fond of a sexy dress or hat. Jane Asher is a well-dressed alcoholic rather than the tragic and mysterious counterpoint she is meant to be. In the midst of all this, Angela Pleasance lurks in black, looking like a witch, and the dreadful Miriam Margolies plays an interfering busybody quite convincingly.
The men fare slightly better. David Jacobi as the quick tempered colonel baffled by modern times is quite good (though he seems unlikely to have been capable of running the wartime operation which is connected with the inserted sub-plot), and Mark Gatiss is reflective as the curate. Tim McInerny is acceptable as the somewhat bewildered vicar.
If you want a proper account of the novel, watch the BBC version with Joan Hickson.