In reply to bedsforsleepypeople:
You might find the following website helpful:
http://www.caravanwise.co.uk/motor/diycamper.html
These are the requirements for a vehicle to qualify as a motor caravan:
There must be a door that provides access to the living accommodation.
A bed, which is an integral part of the living quarters which has a minimum length of 1800mm or 6 feet. This can be converted at night from seats used for other purposes during the day but must be permanently fixed within the body of the vehicle.
There must be a water storage tank or container on or in the vehicle.
A seating and dining area, permanently attached to the vehicle. The table may be detachable but must have some permanent means of attachment to the vehicle. It's not good enough to have a loose table.
There must be a permanently fixed means of storage, a cupboard, locker or wardrobe.
There must be a permanently fixed cooking facility within the vehicle powered by gas or electricity.
There must be at least one window on the side of the accommodation.
If the vehicle has all of these minimum features present and permanently fixed and installed properly then you should be able to have it reclassified as a "Motor Caravan". ...
"If a vehicle has been modified from its original specification it is a legal requirement that the vehicle keeper is required to return the registration certificate, V5C for amendment, no fee is required." (DVLA)