Quick update: I removed the small breather pipe from the radiator to the expansion tank. It was clear. Air line on the expansion tank connection to clear that. Using a small funnel inserted into the small vent pipe, now reconnected to the radiator but not the expansion tank, I slowly added coolant to the system through the funnel until the level in the expansion tank started to rise just above the ribbed section. Started the engine and left running until warm and the radiator started to get warm.
So, thermostat clearly working and air lock now out (so far).
Removed the expansion tank cap in the morning and there was a wush noise, indication the system remained pressurised over night.
I then took the car for a nice drive to the coast and back (100 miles). All good!
Let's hope the fix is permanent.
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