Yeah, I had similar problems with the ending. I think the idea was that, by doing what she did, Ruby trapped Mad Jack back within the circle again and so the future PM would never do the terrible thing in our current timeline. But that doesn't make sense, because the Doctor had heard of what he did before the circle broke (I went back and checked!).

Oh well, it was still a really enjoyable episode. Boom and 73 Yards have been a welcome return to form after a string of really silly episodes. I don't object to silliness in Doctor Who (it's always been there, and the whole concept of the show is silly), but there had been too much of it on the trot and it's been nice to get back to being tense and scary.