My daughter's VT Commodore has 'died' for no apparent reason. She parked in the driveway & locked it with the key button, but it made a funny sound as it locked & then wouldn't unlock with the button. Eventually we gave up using the button & opened the car with the key. When she tried to start it again, the motor gave a slow whirr (like we had a flat battery...which we didn't)& refused to turn over. The Mode light was flashing the whole time so we assumed it was the immobiliser preventing the start, so we did the key in the ignition for 2 hours to disarm it but only got the same previous result...so its not the immobiliser. What else can it be? Was running fine, no misses or probs but now its dead. Can anyone suggest any solutions or diagnostics please?

Have you tried measuring the battery voltage. A dying battery with low voltage will present the same issue you're having right now. Try temporarily swapping it for a known good battery from another car.