He could have used a 1.3 shell (for example) with number plate A123BCD. Tranfered all of the S1 equipment over, interior/ exterior/ mechanics, etc then put the S1 number plate & VIN onto the 1.3.

Or, as said, he could have stolen a S1 already, then changed the VIN and number plates to match the scrapped car.

Read this page for info on re-shells and how cars keep their original number plates and how you go about registering it legally via DVLA:
http://direct.gov.uk/Motoring/Buying...246&chk=VsQ/Fs

If YOU reshell your car, you may still have the original shell, but you cannot legally transfer it's numbers across. That's a ringer.