Renault Master immobiliser stays off, engine turning over but not firing

I have a 2003 Renault Master, which was left unused for about 4 months.

The little light on the dashboard (between STOP and SERV) does not even go on.

When I go to start it, the engine turns over but it does not stay on (as in the engine starts and then dies).

Also, I have charged the battery overnight and it is now fully charged.

When I first turn the key, all the lights on the dashboard go on, apart from the immobiliser red dot – which stays off. I am sure this should stay on until the engine is going…

Any ideas on what it could be?