Sorry - I have only just read your comment on my response re arriving at Caen ferry port. Chat Martragny do take camping cheques but if you check in the book I think that their low season does not start until end of August. We just pay with cash / card. Just checked - they accept camping cheques from 1st May to 30th June and from 1st - 12th September when the site closes. Cannot see any special deals.