I think it is going to be difficult to get on to a proper site at that time of night, Jon. Your best bet may well be to try one of the 'Aires' on the A16. Although some of the 'Aires de Reste' (basically very good picnic areas) are pleasant, for the sake of security, I would suggest looking for an 'Aire de Sertvice'. I think the first one you will reach is the 'Aire des Deux Caps', which I don't think is more than about 10 miles from Calais. If that looks too busy, keep goiong for a while - there are several others. Unfortunately, these tend to be full of trucks and not very peaceful, but if you are tired, you will probably sleep through a moderate amount of noise. We have stayed overnight on the quieter ones and have never had any problems. However, with all the scare-stories around, one hesitates to actually recommend those to anyone.