P38A Immobiliser gone all fussy

Hi all, my 1996 4.0 has suddenly got all immobilser fussy. I used to be able to open the car and start the engine no matter how much time had elapsed since the immobiliser was operated. Now it seems that any longer than the time to think "I wonder if it will start" and I have to re-zap (not sure that's actually a word) the immobiliser.
It hasn't had the battery disconnected or anything similar; I haven't got the "key battery low" message or any other errors (apart from "ABS fault" and that's the sensor needing to be replaced - I know that for definite becuase I had to cut the old one off 2 weeks ago).
Any ideas?

Turn off passive immobilisation and everything else with your Nanocom!

Change the battery anyway, it's a cheap experiment if nothing else.

Col

Car battery voltage low?