Brivis Cooler not working

[ozzie_fred]

I have a Brivis combined ducted heating and (refrigerated) cooling system. After a power break I can no longer select “cool” on the Wall Controller.
Heater is MPS HE20i XA V2, Compressor is CDU11102-C240, Wall Controller is NC3-Rev06, Ceiling Controller N-G1/lo Heating Module.
I’ve reset the NC3 but still can only get “heat”, ie the M button does nothing. The M button contacts are clean.
The compressor is OK as it responds to an externally supplied 24Vac start signal, but the N-G1/lo doesn’t send it the 24Vac.
The N-G1/lo does not seem to have as many items on its display as it used to have.
Any clues as to what I should be looking for/resetting/replacing?

[wotnot]

Can only find some service manual stuff...