You are correct Myles, I dont think it would be much of a car evening, more the xmas social and the meal, so would probably not even bring my car, what do you recon?
How about we stay a a travel lodge or similar and just dump the cars there and forget bringing them to your restaurant?
Cheshunt seems to be closest:
http://www.travelodge.co.uk/search_and_book/hotel_overview.php?hotel_id=339
and it has 80 spaces.