Prettily shot, some nice period detail, stylish... but not Christie.
Tommy was a little dry for Tommy, but otherwise inoffensive.
Tuppence -- completely out of character. Not that it was a bad performance by Ms Scacchi, obviously within what was in the script for her character; but just not Tuppence.
I could see why it was written that way, they needed a contrast between Geraldine McEwan's Miss Marple and Tuppence. But it was all wrong.
So beginning with some jarringly bad misconceptions about a character was not a good starting place.
I know they have only a certain number of Marple books to play with, and they want to keep the series going, but taking the character and inserting her into Christie's other stories (Towards Zero gets the same treatment) is to the detriment of both the character and the stories they are re-writing.
This one particularly suffers from an imbalance, the story has shifted off centre, away from the actual central character and split between two characters. It's a shame.