I had quite a few stolen recovered vehicles when I was in the UK and yes the no. is correct and has to be stamped on the shell somewhere on the drivers side front (normally the strutt tower) but these days will as far as i am aware carry a Q reg only.
These cars are Q when the original id is lost or removed while being rung.
These chassis no are aslo given to cars built from parts. Kit cars etc
The guy if genuine should be able to show the DVLA letter instructing the chassis no to be stamped by a garage. You should also ask for the receipt showing who did it. But should be on a Q plate
If the vehicle is recovered and is still traceable to its original id / reg by chassis no or engine no then it retains its reg and chassis no s
I know this because I use to do cars for insurance companies
ie when recovered by police if identfiable they become property of ins. company who normally want them back on orginal id to get top $ when sold to recoup there pay out.
I don't think it has changed but I have been out of country for couple of yrs now so may have.