After some research, I think I've come across the cheapest and most flexible way to cross the Channel. I used DFDS this year, using their NEC offer and booking through Caravan club, but both avenues are now considerably more expensive. MyFerryLink [Sea France as was] sells 'carnets' of between 6 and 24 single journeys. Prices start at £31 and come down to £25 depending on size of carnet - there is a 25% surcharge for motorhomes, which meant that my 6-journey carnet came in at £232. They are completely flexible - valid for a year from the first journey booked. Supplements apply if used on some very popular dates and times, which are listed. But otherwise I reckon that three return crossings for £77.50 each is pretty good value. See http://www.myferrylink.com/ferry-offers/dover-calais-ferry-offers/carnet?packedargs=site%3DSF_Pax_Uk if interested.