Thanks to all for your help.
Now up running.
I tried rotating the plastic outer ring, it came away
in my hand revealing an amber coloured ring surrounding
the ignition key hole.
Although the amber ring felt quite solid, I gave it a
firm technical jiggle.
Tried both keys and now working ok.
So the normally slow flashing immobiliser light flashes
rapidly if the ignition is turned on but the key is not
detected, goes solid them diminishes if a key is detected.
