The thing that was niggling me was the fact that there was no voltage across the Active-Neutral terminals of the pump when I checked.
Monitoring the terminal voltage I have checked whether the pump is activated immediately the system is powered on, and I found that it is not! No voltage at the terminals.
This could point to a relay problem.
However, before looking at the relays, I thought I would do another test as follows:
Ambient temp here is 15degC (yes it is summer!).
I turned on the AC in the saloon, and set the desired temp to 17.9. (The lowest possible setting).
The AC produces some slightly warm air, but the seawater pump does not start.
I then used a hot air gun to slowly raise the temp of the AC panel in the galley.
When the temp reading on the panel reached 18.9 degC THEN the seawater pump started.
Conclusion: The seawater pump does not start until the ambient temp (as detected at the panel) rises above the set-point (desired temp set by user).
(I guess this corresponds to the compressor starting up).
Most folks might think that the pump operation is always immediate. In hot weather, it does, but in fact it depends on ambient temperature vs set-point.
Thanks for the pointers Olivier, Ruedi and Jose.