Can recommend Camping Chateau du Gandspette. It's just off the direct road between Dunkerque port and St Omer, about half an hour from the port. We have used this excellent site countless times before returning for the ferry - beautiful French ambience on site to finish your holiday.
We have left the site very early - no problem as there is no barrier.
Agree with this suggestion - friendly site, good inexpensive restaurant so you don't have to cook on your last night - and good clean 'facilities'. There's also a small 'Casino' supermarket half a mile away, where you can get all your french goodies without the huge queues in Calais! We stay there almost every year - should have a season ticket!! Just ask for a pitch you can leave early without waking your neighbours!
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