Interesting that it says Matt's now bumped up to the 13th Doctor, rather than to the 12th. I wonder if that suggests the Valyard's around somewhere. Or is there yet another unknown Doctor?
This is explained in the latest DWM - it's fairly straightforward reasoning that is being used, but I won't say unless you actually want to know.
Anyway I came here because the first clip from the ep has been released. If you want to go in knowing nothing, don't watch, but honestly it doesn't give anything away.