Phil
2 things for you to try :
Firstly check the battery in the key fob is not past its sell-by-date. I had that a few years ago , and on replacement it was fine.
Secondly try pulling the immobiliser fuse and see if that cures the issue. Not sure which number it is on your +4 , but I am pretty sure that MMC were using a Rover-sourced immobiliser at the time (from a Discovery?) and it will be wired in a pretty standard manner.
On my 1999 +8 the immobiliser will drain the battery to a non-start level in about 2 weeks, which is why I connect a top-up charger/conditioner at all times whilst in the garage.
