Aussie_V8:
i need some way of the arduino sensing that the car is running, and thus switching off the ignition relay
you mean start relay?
If its petrol you could hook into the coil and read rpm. when it starts the rpm would go from cranking speed up to idle speed. diesel must have a sensor for the tacho, so you could tap into that. Also you would want it to check if the engines running after the 1200ms anyway, and kick it in the guts again if its not.