I do remember thinking Geraldine McEwan came off best as Marple - and that despite the changes to the original Nemesis plot the climax was still very good, so it's probably time for a rewatch. I've been trying to make my way through the older film adaptations of Christie books that I haven't seen before rather than rewatching lately - I made time for a rewatch of the Marple WDTAE after seeing the new one out of curiosity since I hadn't seen it since it originally came out and I thought it couldn't be as bad as I remembered, but it was in fact worse :( The plot changes really do turn it into something entirely different and in general I think it's a bad one to insert Marple into anyway since the story is supposed to be about Bobby & Frankie having a different main makes them less necessary.

I'm glad to have brought the new WDTAE to your attention - very weird that BritBox hasn't shown it to you yet if you're already watching a lot of Christie stuff on there, but I have been generally wondering about whether they've promoted it enough since I've had to tell a few people about it who I'd have expected to know it existed at least. I hope you enjoy it!