This link http://www.dfdsseaways.co.uk/ferry-routes/ferry-to-france/NEC/ will take you to the DFDS booking page, where you can book a ferry crossing on the Dover/Calais/Dunkerque routes for very little:

Dover to France - Low Season

Car + 4 Each way £19

Car + Caravan + 4 Each way £29

Motorhome + 4 Each way £24

Dover to France - High Season
Car + 4 Each way £29

Car + Caravan + 4 Each way £39

Motorhome + 4 Each way £34

and free amendments up until the 30th June (subject to availability).

Worth it for all those of us who haven't yet booked a ferry crossing for this year.