Thanks for all the hints. I continued to have problems with the CH until I found a proper engineer who, unlike BG's finest who hadn't even looked at it, correctly identified that the pump was failing and it was that which was causing kettling in the boiler and F24 faults. New pump and the boiler is now happy. He also changed the leaking manual air vent valve for an automatic one- now protected by the stopcock should IT ever decide to fail. Since I was still just inside the cooling-off period I did tell BG what they could do with their service contract.
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