Sometimes it's on a sticker inside the manual, if you can't get one, go to the local dealerhsip and they will get one for you. I think they charge though.
find out were the car was bought from new, look for dealer stickers in the rear windscreen or on the number plates, then contact them with the chassis number or vin number ( they are the same thing) you should find them on the tax book you got with the car, its a long string of numerals and digits, give this number to the dealer and they may have it on record if not you will have to pay for the records to be looked up. usually about 25 -30 £