Alasdair - what dates are you looking for? I presume you're looking for early October? (I hate the campsite websites which don't actually tell you on the homepage, basic stuff like opening dates, number of pitches, etc).
There are some topics on here: http://www.ukcampsite.co.uk/chatter/search.asp?search=Calais+all+year&searchMode=allwords&searchIn=Topic&forum=7&searchSort=dateDESC&Submit=Start+Search
which might help.