I assume an 98 GTI Escort then?

It will physically fit but it won't run the car due to the immobiliser. The key is matched to the receiver on the ignition barrel which inturn is matched to the ECU...

I am half sure the immobiliser is part of the ECU on these cars.

Have a look at your ECU and see what number it is.....there are loads of variations to the EEC ECU....