It is, I believe, possible to rebuild Cat B on a new chassis (where the vehicle has a separate, distinct chassis). However, DVLA will issue a new chassis number. The rebuilt vehicle may require engineer's inspection (certainly will need a safety inspection) and a new reg number is issued.
Hi Graham, the question remains, if MMC wouldn't sell a chassis for this car, who did (if anyone)? My suggestion would be that it was repaired, not replaced.
