Thanks everyone,
Your fast responses are most welcome.
Planenut - I have PM'd my details
Richard Wood - Thanks I have a copy of the wiring detail - the fuses are OK, the power to the immobiliser seems to function OK, its just that it doesn't seem to be releasing the starter motor.
Obie - the transponder seems to work, I do have to wave the fob close to the steering cowl antenna - its always been this way.
RobCol - yes I have tried the spare key/fob.
Nippymog - Also yes - this is the starting procedure I have always used (3 steps Ignition on, fob to disarm - you can hear it click, press the fun button).
Fingers crossed on planenut's bypass plug.
Will let you know how I get on.
Thanks again everyone.