Interesting topic.
Not sure if this helps or is relevant but a brief as I can;
I bought a ex works Morgan Plus Four in April with around 13500 on the clock.
Now has just over 15000.
Middle of June was a weekend tour of around 400 miles in searing heat and the car was impeccable.
However on the way home it suddenly overheated.
With the gauge in the red I was rescued by the RAC who topped up the header tanks, (the cap of one did not appear fully tightened).
RAC man run the car to operating temperature with the caps off to make sure no airlocks and I set off again as all seemed well.
After an hour running in heavy traffic the car overheated again.
Managed to top up again and drove home very steady.
Next morning no water in header tank on nearside. (I am certainty not technical and was told later the offside header is something to do with cooling the turbo).
Garage were tremendous, picked up car and established problem was airlocks and the system had to be drained and refilled in a specific way numerous times.
The level in the header tank is evidently critical and should not be over filled.
Done about 300 miles since and no issues.
Any work on the cooling system would appear to need detailed knowledge.
