I'm sorry that my advice seems foolhardy. Last time we went through Calais in September last year there must have been 50 vans staying on the docks on a Friday night - it was a real community. Similarly on the Aire's we've stayed in there are always many more caravanners. Julian did ask if there were areas - and yes, there are. Whether you would recommend them or not is a different matter.
I don't know why Julian is crossing at night, but I know why we do. We start our holidays after we finish work on a Friday - drive straight down to Dover from the North of England (sharing the driving) and then get a midnight(ish) ferry. We choose to stay as near to Calais as we can - that way we feel as if we are in France from the very first day. If we are early we get a site, but if not then we stay 'wherever'.
I know every year there are stories of gassings, etc., but we've stayed in Aire's sometimes, going back over thelast 25 years, with no problems whatsoever - and indeed at some our teenage sons have joined other teenagers in their tents on the grassy verges. One 'Aire' near Boulogne has coin-operated hooks ups available, and a shower block. Despite all the postings on this site no one has ever actually said they have been subject to a gassing - it's always someone they know, or a friend of someone they know.
Again apologies, but each to his own, and if Julian wishes to stay on the docks then at least he knows there is an area where he can stay and if the overnight ferry was the only one he could book in on then he won't be alone on the dockside in Calais - we might be next door to him.