Showing their support for the Children In Need appeal (go and donate) the BBC treated the audience to this two minute preview of the Christmas Day special. If you are dead set against spoilers, there is a bit of one so don’t watch. It’s Santa Claus and a couple of elves, but is all as it seems?
https://www.youtube.com/watch?v=MsxEenCBRG0
Just because I’m nice, here is the 30 second trailer that ran after the series finale.