Cooling – possible head gasket leak or crack unlikely

Seems this cooling issue has been my biet noir since the start. Hindsight is a wonderful thing and I should probably have taken advice, sought help from the start. On the plus side I am fairly intimate with the system now and have a good chance to keep on top of it. Jan suggested, as a MOR position to do a combustion gas test on the cooling water to detect any leak. I conducted this first thing today and couldn’t detect any gas so that is good news. Might still leak gas under load etc so I’ll keep the test kit on board and try again at some stage.