In reply to chummer:
!!! Just looked up the price difference on norfolk line, on a sample quote (ie, what was already in the boxes) it's 4x the price to be a van than it is to be a motorhome of the same size.
On their website it says
"A van is defined as a motor vehicle which has no windows in the rear and is designed for commercial purposes, irrespective of commercial or private use;"
It doesn't really define a motorhome but suggests under 'motorhomes with trailers' that a motorhome should have "windows, sleeping and cooking facilities".
http://www.norfolkline.com/EN/Ferries_to_France/Ferry_help/vehicle_types/
I imagine it varies by company then, I realise it wasn't your intention but thanks for the heads up, I shall have to check terms before booking a ferry in future (mine is registered as a light goods vehicle). I forget that my van isn't really a car sometimes.