Rupert, appreciate you advising from your sick bed!
There was definitely water under the rocker cover, and it's emerged via the rocker cover gasket and around the rocker cover bolts.
There appears to be neither water in the oil (we drained the sump), nor oil in the coolant - no milkiness in either, the only milkiness was in the head under rocker cover. I think the oil may have had some diesel in it. The radiator had been filled recently, and the water level was not appreciably lower.
Thermostat is working. Hoses, thermostat cover etc all seem to be OK.
Leaving checking head gasket etc until we absolutely have to!