Well, it's fixed.
The cam cover gasket and plug seals were leaking and were replaced, the spark plugs were oiled up, and the crankshaft position sensor was deteriorating. The first two items were most likely the cause, but the sensor was likely contributing to the problem and there was no sign of water in the oil, no loss of water or loss of oil. So the problem appears to be fixed.
Annoyingly, I think I could have tackled all three little jobs myself had I not panicked and assumed the worst (head gasket) and had a mountain of other things to do at the same time. It was a sudden and puzzling problem that seemed to come out of nowhere. I had noticed the leaking cam cover but in my naivety I didn't think much of it
Hey ho some you win, some you don't. At least it's fixed. Drives like it should at (legal) revs and speeds now. And as a conciliation for being a bit too eager in diving it and fixing things before advising me of the findings first, the garage replace the crumbling top radiator bushes for £0.
Thanks to "the panel" for all encouragment/ ideas/ tips etc. etc.
One day I'll start working on the red one.