You think on it for a moment. "Uhm...a suit?" you offer, badly disguising your uncertainty.
Chaps are often stumped by Christmas shindig decorum; is it acceptable to wear office attire? Is it a time to dress down? What will my boss think if I slip into a pair of jeans?
My theory is that an office Christmas party should be treated like any other professional social event. Not making too much of a fuss of it and not using it as an excuse to dress like you did at the dingy student union.
The best option is to wear a suit - and use the one you were wearing that day. It's OK to take another suit if you want, or if it is required (e.g. Black Tie) but there's something rather charming about going from one environment to another in the same attire. It's a good idea to choose a suit adaptable for the occasion. I used a burgundy one here, available from Marks & Spencer.
This is one idea: stuff an ivory silk evening scarf into your briefcase, remove the waistcoat, tie and pocket square from the 'day' outfit and tie said-scarf artistically around your neck for a still-elegant but fun-seeking, looser ensemble.