When you stop to think about it, the disappearance of Agatha Christie seems such an obvious Who plot opportunity. There were some ropey ideas - Professor Peach and the Lead Piping for one - but somehow I'm inclined to let them get away with it. The idea of the Doctor being poisoned was a good one; somehow I can't think the demand for Ginger Beer as an antidote was selected randomly, given the goings-on between some members of the household. The continuous stream of references and puns was fun to track. Probably not as good as The Fires of Pompeii, but better than Partners in Crime or The Doctor's Daughter.
And next week ... "the biggest library in the Universe". Would that be the New New New New New New New New Bodleian?