Sounds like an alarm/immobiliser problem to me. Presumably the central locking works from the remote fob? Have you tried the EKA code (key left and right in drivers lock so many times). Early Disco II's had problems with interference (parking near aerials/transmitters etc etc).
A friend of mine rang me a few months ago saying she couldn't start her Defender. It never gets locked and she didn't even realise it had an alarm. Replaced the battery in the fob, alarm armed and disarmed, car started. . .