97 pug 106 wont start after switch wrecked - Transponder?

[Philjw]

Can anyone tell me where to find the transponder aerial in the pug 106 and what it looks like?
A helpful breakdown man has wrecked the ignition switch on a friends car. I think the problem could be the immobilisor. I have a spare ignition switch from a Xantia. This fits the plugs and seems to power everything but the engine won't start.
There is a black plug hanging loose near the ignition switch with nothing to attach to it anywhere to be seen. Is that for the transponder?
How should I test if the immobilsor is the problem? Does it kill the ignition - there is no spark at the plugs - or stop the fuel? Other posts have mentioned a double relay to be found under the ECU. Could that be the problem?
Any help gratefully received. I'm more at home with Citroen diesels