Bazz, try this - not a comprehensive site guide, but may help - http://users.pandora.be/leo.huybrechts/camp1.htm. It's actually a compilation of sites near the main routes through Europe, but does give opening dates - so you can look for those open all year. Site details are also given, so you could try a quick phone call. See also the campingcheque.co.uk map for sites - I think a few of the Belgian ones are open all year, as are some in Germany.
Try also http://www.campingfrance.com/index.jsp?lg=uk which gives details of all sites in France - but is not so easy to find those open all year.
Hope you have a very Happy New Year, and find what you are looking for. If not, then there are lots of 'Budget' hotel chains in Europe, where a room costs not much more than the fee for a night on a campsite. Try http://www.eurapart.com/budget.html for the list, and links to all websites.