This trailer aired after the Christmas special today and engages with some of the major plot points of that episode. You probably wouldn't want to see this until after The Snowmen unless you're really not afraid of spoilers.
For some reason, this is starting ten seconds in for me. Just pause it an restart it.
I expect that embed doesn't work in the US. I'll do my best to find an unlocked version as soon as possible.
UPDATE: I hope this was soon enough for you. The picture quality isn't as good as in the official one, I'm afraid.