I've just remembered - your vehicle (depending on date of manufacture) might have a VIN (Vehicle Identification Number) visible through the windscreen on the top of the 'fascia' (not the instrument binnacle - the sheet metal of the cab structure) which will look like a 'dymotape' strip (though metal). This might provide information (such as the chassis number) that might be missing from the model plate that you found under the seat.
It's a complex number and not easy to decode:-
http://en.wikipedia.org/wiki/Vehicle...rer_Identifier