Try Chateau de Ganspette at Eperleques, just north of St Omer. They take overnight bookings - but in August you may find you need to ring up to reserve first. There aren't that many 'recommended' sites if you look in the Caravan Club or Alan Rogers guides in that area, and some seem dubious quality, but Ganspette is lovely, friendly, clean facilities, good size pitches, and a really good value restaurant. We've just had our usual overnight stop there, and had Tarte Poireaux (4 euros), a Salad Verte (2.5 euros), Steak/frites (9 euros), and a carafe of wine (4 euros). Can't be bad - and they fitted us in after we arrived at 10 past 9 at night!!! Can you imagine any site in England doing that?