Reporting on what I found in the carb: the main jet / emulsion tube all looked ok; viton needle very slightly shaded. The pilot jet was clean, but the pilot (idle mixture) screw was badly carboned / dirty. On cleaning, the taper was not badly worn, but had what I would call mild pitting / streaking to its surface. I fitted a new float valve and pilot screw, and made sure all the passages were clean by running IPA with low pressure air. The result is a much improved tickover, with only slight 'creep' upwards in rpm from a cold start. So, well worth doing.
I think this sorts out the overheating cause & effect. Moral: look after your water pump. Thanks again to RCW!
I have a couple more questions, but a new thread is called for......
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