Not yet, but the shower is used only rarely so we haven't rushed to do anything about it. I've not been able to track down replacement parts at a sensible price, so we'll just replace the whole unit.
I'm curious to investigate further though, so once the old shower is out I'm going to break open the canister for a closer inspection.
Paul, If it's not used that often. Disconnect it, take it home and dismantle it. It sounds like an Electrode Boiler unit that is all scaled up. Scale will kill an Electrode Boiler, as in more resistance. Hence cooler water. Get Emery paper on the probes.