rover 214 - alarm immobilised how do i get the code - j0hn b

my fob opens and locks ok everytime but the engine is immobilsed , my book tells me you can use a 4 digit code via the key in the drivers door . i have my security card which has a hand written number on it but only 3 digits not 4 ,,,,could it be a 3 digit code or has some idiot only written down part of the number