Immobiliser light stays on

Did you check to see if the fuel pump relay clicks when you turn the ignition to ON?
If the fuel pump relay does not click; some vehicles have a fuel pump safety reset that may need to be reset. Not sure if your vehicle has one or where it's located.
If the engine cranks over quickly, but does not start.... then the ignition system problem is not battery voltage related.
You need to start with the basics to determine why it wont start.

Sorted. The battery man had an idea. Fit new battery, lock up the car and then unlock. Job done. I assume that it was all to do with things being in the wrong sequence. A bit like trying to plug a mouse into a computer that's already switched on. Thanks for the help..

Nice one. Did you try to start it straight after he fitted the new battery before going through the door locking procedure?

Yep. New battery, light stayed on, engine turned, nowt happened. Switched off, locked and unlocked doors and it worked perfectly.
Now what dimwit thought it a good idea to let such a problem happen. Mentioned it to my local garage this afternoon and was told that they have had a similar problem involving Vauxhauls and changing head lights

Modern day electronics. Even the inventors don't really know how they work!!!!!

I suspect what happened was that the battery voltage dropped too low and the ECU or immo reset.

mattsccm wrote: My Land Rover doesn't do this.

My Landrover (a Freelander) was a nightmare over flat batteries because the rear window winds down when you don't expect it to. Hated it.