Headrush
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It does sound very much like a circulation problem as you already suspected. Maybe the pump runs but is just weak. I've replaced many pumps in various systems that appeared to work fine but didn't have the strength to circulate round the rads.
Another thought (however unlikely), has one of the flow or return valves at the boiler been inadvertantly closed off. Maybe they have even broke. Again I've seen valves that seem to rotate on the outside but nothing is happening on the inside.
Does the flow pipe from the boiler heat up quickly? How far along the pipe does it heat up? If the answer is yes but not very far then you either have a weak pump or a restriction in the circulation. If the pipe heats up gently from cold, that's because it's drawing plenty of cold water from the return, assuming all else is well with the boiler.
When you run the DHW, can you see the diverter valve moving freely? Does it back off fully when you close the tap? I'm clutching at straws here but there could be all manner of things causing a circulation problem..
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