Sunday Reprieve – First job was to flush coolant system with a cleaning treatment, followed by two clean water flushes. Then refilled with just water, careful to make sure that level in rad only just covered core. Then spent an hour running the engine, driving and taking lots of temperature readings with the infra red thermometer. I tried very hard to make the car overheat, long periods at idle, followed by simulating driving in heavy traffic. After an hour and with the temperature gauge showing 100 degrees (C) I stopped the engine. No boil over into the header tank.
Weekend Conclusions: The temperature gauge reads 12-15 degrees hotter than reality, add this to a general paranoia about overheating and blown head gaskets and you really have a problem. The level of the coolant in the rad is critical, too higher level of coolant and when the engine is turned off when hot the car will boil over into the header tank. With the correct level of coolant, it circulates better, runs cooler and does not boil over. Next step is to do some road miles to confirm this. I had already order separate temp/oil pressure gauges to replace the Smiths dual temp/pressure gauge, I’m hoping that a dedicated temp gauge will be more accurate and reliable.