Thank you Cobber! A few things to think about. I didn't realise there was a correct sequence to bleeding the cooling system. Will do as you suggest in the new year and report back if I get a result. Just praying it isn't the HG, I have heard that a tiny crack in the gasket between water jacket and combustion chamber might produce the symptoms I described - with no other obvious outward signs. And that the engine might keep going like that for quite a long time. Very risky though because the HG is bound to eventually go - leaving me stranded somewhere no doubt!

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