small update: having taken the old Stop Solenoid off, i did a brief test with a test battery. The first two times i connected it, nothing happened. Third time, it activated and pushed the plunger. Whilst this is not proof, it does indicate that the issue was caused by a sticky solenoid. Hopefully, when the replacement arrives, I will be able to stop the engine.
Next question: whilst i can't do a proper test yet, I have a feeling that when i get the engine going, the "code" red warning light on the dashboard will stay on. I am wondering if there is a simple way to disable the light:
On the connector that went in the immobiliser
1) i am using the middle one to connect directly to the solenoid (as my test shows this has current when ignition is turned on)
2) i have sealed up the other 2 cables. I wonder whether there is something i can do with these which will suppress the warning light?