I have found ANOTHER link to 2009 Poirot Three Act Tragedy which has been reposted, I can't promise how long it will stay up. Part 1 here
ITV had a special where they interviewed David Suchet of his experience playing Poirot through the years. I would agree with his selections of some of the strongest performances he has encountered. Elizabeth Dermot Walsh (Elinor Carlisle) in Sad Cypress (a favourite!), and the entire cast on Death on the Nile are among the very best.
Poirot & Me part 1 watch here (link will go to part 2 etc...)
Julia McKenzie interview here
Oddly enough I've stumbled on a strange version of both Poirot and Marple characters in Japanese Anime (here's a taste). It's definitely not my taste, but it is interesting.
Marple played by Geraldine McEwan
2006 Sleeping Murder part 1 here
2005 A Murder is Announced part 1 here
2004 The Body in the Library part 1 here
2006 Ordeal by Innocence part 1 here
Marple played by Julia McKenzie
2008 Why Didn't they ask Evans part 1 here (Natalie Dormer, Tudors... this link may not last long)