landy in the blood

Hi i have a 51 plate disco 2 and having problems with the central locking as the fob will only work with the key touching the door lock outside not ideal as the immobiliser won't work either. Sometimes if you sit in the landy a few minutes when you go to start again it's immobilised and the alarm goes off needing you to open the window and put the key on the lock outside to stop it. I have tried replacing the battery but it made no difference at all anyone have any suggestions please