I think we have now proven categorically that the HW sensor lost comms is actually a radio message that the controller doesn't pick up from the Hot water sensor, for some reason.
Removing the batteries, tightening the battery terminals etc on the Hot water sensor appears to fix the problem because the Hot Water sensor transmits a message on startup that the Controller picks up.
I've even hard wired my CS92A, so that it doesn't need batteries but even that doesn't solve the problem.

Yesterday I discovered another way to force the CS92A to send a message to the controller. Just pressing the HW BDR91 button on the relay seems to force the CS92A to transmit a message that the controller then picks up.

So the next time you see a lost comms for the HW sensor, do this first before playing around with the sensor itself.