Hi Louise
We spent a very comfortable night sleeping in our 'van at Dunkerque port on Sunday 25 July after arriving on the 6.00pm boat from Dover. It meant that we were off to an early start the next day. Last Summer we slept there on the way home and managed to get on the 6.00am boat at no extra cost although booked on the 8.00am.
When you leave the boat at the first roundabout follow the signs to ferry and then to "no tickets" (or words to that effect) There is a large car park adjacent to the Norfolk Line building where there is a cafe and toilets. Both times we have stayed there have been lots of other overnighters too.
(Sent from a very hot and sunny Croatia!!)
Tim