Immobiliser fails to recognise jackplug - no start?

A '93 Nissan Micra with what appears to be an aftermarket immobiliser in the form of a socket beside the ignition switch, and a three segment jackplug.
Ordinarily a red LED flashes slowly when immobilised. Inserting the jackplug activates the immobiliser and the LED flashes rapidly for one minute enabling key insert and start.
Currently, inserting the jackplug has no effect on disarming the immobiliser - no rapid flashing of LED occurs. There is a separate fuse for this device, and it is fine. What next? Car unusable. Circuit diagrams in Haynes show no circuitry (why would it!). Stuck - help