Was outlining the process I went through when mine was a Cat C

When you book the car in for its VIC you have to describe the damage/repair. VOSA then pass this info on to the DVLA who add it to the new V5

The notes appear on the front page in the box that says 'New at first registration'.
In my case 'Replaced tailgate, rear bumper and OSR Quarter and repaired D pillar' became ' Vehicle has had substantial repair to significant rear accident damage' or summat along those lines

If you knew the car had been written off before you bought it, then fair enough, but the sellers accidental ignorance has put you in this position, unless they are a trader who bought the car direct from the insurers, in which case they really should have known better

Believe someone else here had a similar problem...they bought a car, all the paperwork was in order, no mention on the V5, but when the new one came back to the new keeper, it had the notes on the front page. Dealer claimed to know nowt about it, as the car had been part exed. You can bet that the previous keeper had filed the reissued V5...
