I think the characters and their relationships ARE what's at the centre of the movie, not a "needlessly convoluted mystery" - and I felt the movie did this very well indeed, which was my main concern. It's an emotional, personal story, not a thriller at heart.
I have seen the even better masterpiece of a TV series, though, so obviously I knew more about the characters and maybe gave the movie some goodwill knowing what I did about their stories. And sure enough, the movie has a narrower perspective, honing in even more on Smiley, and our teacher, and giving less time to some of the other key secret service people. It had to, unfortunately, with only two hours available. The TV series had 7 hours and was still jam packed with development. But as a movie, I think it did as well as could be done, and managed to be emotionally rich as well as stylishly shot.