Geoff,
Good suggestion from Lutz, the "Groene Camping / Green Camping" along the Belgian and southern Dutch coast are good sites - they usually have ACSI or ANWB (NL) or Alan Rogers recommendations.
There are a whole load more campsites along this coast, but many are organised like holiday camps, so I think you will enjoy "Groene Camping" better. But always expect around 50% or more fixed, 50% or less touring pitches.
Robert