After all that, the main problem seemed to be a combination of the boiler being broken - water in oil tank and so pump ruined - and a seized circulating pump, which was persuaded into action with a few taps from my trusty lump hammer.
Found myself a good young plumber and all is now running as it should. We are warm for Christmas.
Thanks for all the hints and tips everyone - especially Matt1e for his patience and perseverance.
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