[Neil Jones]

It's probably been asked before but are there any of the Christmas specials missing per se?

Aside from what is known to be missing (primarily the first two pilots and a smattering of episodes from pre 1986) apparently the 1987 Christmas Special of which you mention was missing, as Challenge put out their own appeal at the time:
www.facebook.com 

Though it has since been "found" (though more likely it was never "lost" in the first place, probably had the wrong label on it) but I don't think Challenge have shown it. Nor the 1988 special, but they have shown 1984, 1989, 1991 and possible 1992. The 1990 special was when they dressed up for it as characters from Dickens, and there was an interesting article in the Black Country Bugle this week about people who were dragged up to Nottingham for the 1990 special to be in the audience, and everything that could go wrong en-route did!)