[ajmac]

There used to be a points system for retaining the reg on a vehicle in the UK, it went something like this:
Body: 0 Points
Chassis / Unitary Body: 10 Points
Axles: 8 Points
Engine: 1 Point
etc...
You had to reach 20 Points of some figure in order to retain the ID. However I think that has gone now...

The DLVA state:
Vehicles that have been rebuilt using a mix of new or used parts
In order to retain the original registration mark:

cars and car-derived vans must use:
The original unmodified chassis or unaltered bodyshell (i.e. body and chassis as one unit - monocoque); or a new chassis or monocoque bodyshell of the same specification as the original supported by evidence from the dealer or manufacturer (e.g. receipt).

And two other major components from the original vehicle - ie suspension (front & back); steering assembly; axles (both); transmission or engine.

So, we can summise from this that if a fabrication shop builds an entirly new Carrier monocoque for you from an original pattern this would still enable you to retain the vehicles details and register it as such, so long as you have the axles and and steering gear!

If that's the case for cars (do tracked vehicles sit in a loop hole?) then all the road legal Carriers in the UK ATM are in the clear.