Just thought I'd post because some of these guys are really tricky when trying to hide the fact that the car is stolen.
You have to check the VIN on the registration disc (that piece of paper in the windscreen) and the one on the chassis. The chassis VIN plate is located right at the front of the engine but it's on the chassis itself, held in place with rivets, it's got the VIN and engine number on there.
Also you should find out where the engine number is located on a car because you have to check that also, if the car has a stolen engine they'll take it to the impound yard and rip out the engine. Both the VIN and Engine number must be checked.
Anyway take the registration number and VIN to your local police station and have them check it out (they have to check it out).