Porsche 930 (1989) H&P Immobiliser Failure ?

[carrpet]

Hi there,
I could do with some help please !
The issue is that my starter motor is not turning when I ask the car to start.
On investigation, there is zero volts at the yellow wire terminal on the starter solenoid when the key is in the start position. There is therefore no solenoid click when key is turned to start.
The car has a Hamilton & Palmer Matrix III immobiliser fitted in 1995, which is installed behind the plywood removable panel in the passenger footwell.
The H&P makes all the right beeps when "key" is inserted but my suspicion is that an internal relay is faulty.
The battery, which is on trickle charge is at >13v as measured at the high current supply terminal of the starter motor.
All of the wires leaving the H&P unit are black and are going to be difficult to trace, so I wondered if you could help me with the fault diagnosis, before I start a major task ?
The car is one of the last 930's made (G50 gearbox etc).
Any help would be appreciated.