New radiator cap rubber seal fitted and water system flushed with Thermocure Evaporust. The cap does seal much better, but my problem still persists.
I think the thermostat itself is at fault, either sticking intermittently or no longer operates at the correct temperature. Testing in the garage the engine warmed up as normal but then the temperature gauge continued to rise to just above 90 degrees, the radiator top was just aired, but the thermostat housing too hot to touch. I switched off the engine and I could hear boiling water in the block, a sudden loud click, and a stream of steaming water out of the radiator overflow pipe onto the floor.
I suspect boiling water becomes pressurised within the engine, the thermostat finally gives in and opens, jet of steamy water suddenly released into the expansion tank, and the only way out is via the overflow pipe (at least now its not spurting out below the radiator cap!!). It looses about 2 pints each time it happens, so it’s the expansion tank full. The radiator itself then gets much warmer, so water must have started briefly circulating to some degree.
I can’t get hold of spares for the inner thermostat workings, so I think I will remove this part altogether and put back the empty housing (as I will not drive the car in cold weather). Has anyone done this “surgery” and has it been successful with no side effects to be aware of?