@JamesD We have used a couple:
Camping La Fontaine des Clercs (Camping La Fontaine des Clercs). IMHO a fantastic site situated under the ramparts of Montreuil sur Mer which is a lovely old town with bundles of great places to eat and drink. It's a nice site run by a young guy who is obviously quite proud of it. The only problem is that it's about an hour from Calais which might not be as close as you want.
Camping Chateau du Gandspette (Campsite France Pas de Calais - ᐃ CHATEAU DU GANDSPETTE **** - Eperlecques). Another brilliant site less than 30 minutes from Calais. If you don't want to self cater (ie. me and SWMBO), the only issue is that their [good] on site restaurant might be closed later in September and you would have to go to the nearest town (Eperleque) for food - it's probably a bit far to walk but only a short bike ride. I've heard the odd comment from Brits saying that there tends to be too many Brits staying there but IMHO that's probably because it's so good! If you've got time, it's also a walk from a V2 Rocket site left over from WW2 which we found to be fascinating (
Accueil - Le Blockhaus d'Eperlecques - 1ère Base V2 & V1). They also take the ACSI Camping Card which is always worth buying.
I've also heard good things
Camping La Bien Assise which is even closer to Calais but we've never used it so I can't comment further.
Hope this helps. Let me know if you want anything else.