Rover 420 immobiliser override problems?

My key fob to my rover 420 hasn't worked in years - but that's never been a problem before because I have alway been able to override the immobiliser and operate on a key only basis. Yesterday I have had to replace my battery and have, as alway, gone to put in the override code, by locking car and turning the key to lock and unlock etc to the correct EKA code but it just wont work! I know I am using the right code and have never had problems like this in the past - can anyone shed any light???