Additional to Val's contribution, we take ACSI membership available from Vicariuos Books (http://www.vicariousbooks.co.uk/contact.htm). We find that ACSI with Camping Cheques, give us adequate discounted site cover.
With both methods, check out whetyer the sites are open at the time of year you want to use them.
The further north you get on the Costa's, the sites (Although not all) tend to open March or April.
We did overwintered last year and found it wonderful.
Bon Voyage
Barry