Just spoke to Paul at WMSS.
John has been working on the car, the head is off, and being cleaned up.
It seems the head gasket had gone, but there appears to be no further damage. The head is flat, and there is no pitting.
They are therefore replacing the head gasket and thermostat, and doing an oil and filter chainge too.
Hopefully this won’t be ruinously expensive. I am glad no further damage was done in the overheating incident.