My 1999 4/4 Zetec 1800 has a factory fitted immobiliser.
Do you have a small red light between the four small dials?
On mine it pulses red every second or so when ign is off.
I had a problem getting a spare key to work as the metal of the key stopped the immobiliser electronic widget getting close enough to the keyhole to allow the immobiliser to switch off.
A local key co had to buy in a qty of special key blanks to enable the new key to work.They still have them and are in Aylesbury.
If you look at the Morgan key you may see it has been drilled out along the length of the plastic end and filled back in.
Thats where the widget is located.
Howard