Fingers crossed for youMorning after - stone cold. I don't open a hot system, seen pictures of what happens on a first aid course years ago.
It foamed as it came out, which is why I was worried about head gasket maybe, but hasn't shown any signs of bubbles or foaming before, so I presume just pressure release like opening a shaken coke bottle. And no signs of bubbles / foaming as I ran it up to temperature with the cap off after either.
It was almost as if it was pressurised from somewhere else with an airlock which had pushed the coolant up into the header tank and kept it there. But I can't see from the diagrams and my understanding of coolant systems how it could do that. Air rises, so should logically trap at the header tank, or thermostat, or top of radiator. And no sign of any problem with that previously.