I'm now sat in the room with the pump. What sounds like huge slugs of air air being produced from the boiler, passed through the pump and into the radiator, with a lot of noise -whooshing and hissing.
It's as if the boiler is heating the water and then a load of air is being forced out quickly. It only happens when the boiler is hot.
The hot water cylinder appears to be working, so there is presumably not a blockage up from the boiler.
The system is 35 years old, apparently, so a power flushing is probably not advisable.
I may have to give in and call in a heating repairer as I've no idea what to do next.
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