abigail_n told me over at lj that Kitty's full name as confirmed by Imdb is Kitty Winter, who is actually a character in one of the lesser known Holmes tales, The Adventure of the Illustrious Client. Which intrigues me further. And yes, this show is generally respectful to its supporting cast, and thus not likely to introduce Kitty simply to show her up as "not as good as Joan".

MI6: I think mid season we might get a furious visit by someone, or a lingering case, yes, but it won't be a main plot through the season. Just a one shot. Which I'm good with!