Everything back together for a test - the circuit I have been adding was a simple immobiliser which cuts some circuits unless another electronic key is used to disable it. It is auto arming, as soon as the ignition is turned off it arms itself and has the ubiquitous flashing led to show its presence:
The intention is to guard against simple hot wiring of the ignition. If someone starts to take the bonnet off or get in the back of the fusebox (good luck with that rats nest), or simply tow the car then all bets are off.
Hopefully though it will stop a passing opportunist joyride attempt and give me a little more protection than the stock Ford ignition key barrel.
I'm not posting installation details for obvious reasons.