Le Bien Assise at Guines, about 8 miles from Calais, accepts people arriving quite late. If reception is closed the instruction is to go to the bar. The site does not have gates that close so I suppose its possible to arrive later and pay the following morning. Might be an idea to phone ahead just to check they have room.
Steve, we always get the late ferry over to France and use the ferry ports overnight parking facility. Its a secure parking location if you just want to get your head down overnight. You'd have to follow the signs for the car ferry when you got off the tunnel, it would take you back on yourself slightly, but its still convenient for the motorways away from Calais, the following morning.
Does anyone know if there is a similar overnight parking arrangement to the one in Calais in operation at Dunkirk? I am giving Norfolk lines a try next year. If not I will just have to drive back to Calais for the night.