Immobilizer
is the ID40 chip in the key ?
+1 for Immobiliser
BUT
the fob flashes but nothing happens to the car (it's the one that has a key and a separate fob)
The earliest cars didn't have a chip in the key. They used an infra-red transmitter in a separate fob that you had to point at a bulb up by the rear view mirror. Once you had disabled the immobiliser you had about a minute to start the engine before it immobilised again.
My first Omega (1994 L reg IIRC) had it. I think they only used that system for the first 2 years.
They had a Chip in the Key, as far as Im aware all UK Omegas had Chip + Transponder set up's, the Infra Red was purely for Remote Locking and Alarm.
Sounds to me like it could be a Crank Sensor issue, is the Immobilizer Lamp flashing when you Try and start the car?
It could be its still in "wont Start Immobilizer Mode", you could try Diss the Battery (Full Charged), fully remove the Main ECU and leave over night, put ECU back in Connect Battery (or other way around cant remember) if need be enter the Car using Key only and try a start and or re diagnose from there..