Major logic flaw? SPOILER
When explaining the solution, Angela says that Rock knew Liz had to be the killer because no WAY would she not have told her husband about the notes. However, Rock was on record with the inspector saying he'd thought he could handle the notes on his own—he never said, "Oh, God, my wife never told me..." So if he never said that, and if we the audience only knew via a scene between Liz and Rock, where did Marple get that from?
I dunno. As much as I love this cheesy campfest, I do think Jessica Fletcher would've solved it more entertainingly!