... and the culprit was:
the leisure batteries' isolation switch. The wretched thing had been throwing me off the scent by being intermittent. When I'd changed the split charge relay I'd obviously isolated the batteries by switching both the starter battery isolator and the leisure batteries' isolator switches: the latter had made contact for a while, I suppose, and then gone high resistance. Before disconnecting the offending item I isolated the batteries. Then I cleaned up the terminals and exercised the switch - but it was still intermittent, but better than before. As we were still outside Thorn Marine, being drizzled on despite being mostly under the bridge (I chose daylight over dryness), I took in the old switch hoping for a shiny new one as in the image below (from the Yacht Bits website).
But despite finding one with the same mounting hole position, the fat round black bit into which the red key is inserted was too fat. So I vaselined the terminals, put it back, and connected up again. From now on I checked the cabin voltage every time I started the engine, to make sure it was charging: if not, a quick exercise of the switch would sort things.
I was now much happier that I'd found the fault (even if it wasn't quite fixed yet - that's the owner's problem).
Here's another picture of "Elk", at Castlefield Basin in Manchester this time.
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