Bit of an update on this...

with the wire from the PCM to the WOT relay disconnected, there is +4.5v out of the PCM (as per FocusV8 theory).

The system has been regassed after finding a suspect pipe from the compressor to the condenser so that is all good.

ForScan and IDS both show that the heater control module (heater panel) is sending a signal to the GEM module and the GEM module is receiving this signal. The PCM is showing as no signal being processed so is either not receiving the signal from the GEM or is seeing some problem which is stopping the activation.

There are no DTC for any of the modules, including the instrument cluster which apparently converts the various CAN signals between the modules.