Maybe another future version of himself told him about it.. you can explain almost anything this way.
Also, the Vastra and Jenny sentiments seem to be quite widespread, and yes I want too!!!!

I do like all three of them. And I like Beka the way she is, but I do admit I would have preferred Rosto over Dale Rowan any minute. Yes, we'll see, I'm looking forward to Mastiff in any case. October can't come fast enough.