It occurred to me that, when you rank all 23 episodes of "Agatha Christie's Marple" from highest to lowest IMDb-rating, "The Sittaford Mystery" is all the way at the bottom of the list. That being said, 6.8/10 is still a terrific rating many movies and TV-shows would be very happy with, but it's remarkably weaker than the average rating of the series. I can think of a few reasons to explain why the installment is slightly less popular.
First and foremost, the source novel is one of Agatha Christie's most known stories, but it doesn't feature Miss Marple anywhere. It's not the only episode based on a non-Marple novel, but the lead character is usually ingeniously processed into the other stories, whereas she's pretty much the third wheel on the wagon in "The Sittaford Mystery". Moreover, Miss Marple solves the murder without even coming near the scene of the crime. She bases her theories on what others tell her, whilst she's stuck in a mansion during heavy snowfall. I know she's good, but she cannot be that good. Secondly, it takes an awful long time before the murder is committed. The film is over halfway already when the killer strikes, which leaves very little time to guess along for the culprit's identity and motives.
And yet, don't be too alarmed, as "The Sittaford Mystery" is nonetheless a compelling TV-movie with a very intelligent finale. Great performances, as usual, and the most momentous appearance here is definitely Timothy Dalton. His biggest role will always remain that of James Bond, of course, but personally I prefer to see him as a sneaky and callous figure, like he did in "The Doctor and the Devils", "Hot Fuzz" or here in "The Sittaford Mystery".