I just watched the mystery tonight, on the link you so kindly provided. Wow, I'm glad I'd already read the book (a few years ago, but I remember it pretty well). I had trouble hearing everything they were saying, and keeping track of who everyone was, but because I knew the story I could keep my bearings. They chopped and changed the story a fair bit, but you have to, to keep to 90 minutes. I liked some of the changes, not others. I agree, there was no reason to pad out David Suchet that much.
Sayers mysteries: There were two excellent series of Sayers mysteries. The one you mention, Isak, which includes "Gaudy Night" had Harriet Walters as Harriet Vane, and Edward Petherbridge as Lord Peter. I wasn't bright enough to recognize Carol Macready here.
The one you mention, Marcie, with Ian Carmichael, was also really good. I'd have a hard time figuring out which of the two made a better Wimsey. Any Sayers fan will probably be familiar with both, but for novices, I would strongly recommend Nine Tailors (with Carmichael) and Gaudy Night (with Petherbridge). I think Nine Tailors is Sayers' masterpiece, it's so full of the atmosphere of the village, the importance of the Church, bell-ringing, flooding in the fens, and the awful dilemma of some of the characters, and the film does a good job on it. Gaudy Night is also really good--it's got a complicated plot involving some intellectual issues that are handled well.