Well, I've never been so impressed by amateur sleuthery, as on here! It's almost more compelling than the series itself!
FWIW, if you go on the theory that it's usually the least suspected character, it's going to be Mark and/or Baptiste who reveals themselves to be heavily involved.
I totally agree that Mark is not what he seems, although he's evidently not been the direct focus of suspicion as the series has progressed (thus far). I suspect all that will change on Tuesday though...