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It turned out to be a load of air in what I didn't realise was the next radiator along.
There are three radiators in our loft extension, the one which wasn't working then two which badly needed bleeding.
Once I'd bled them and topped up a bit with the filling loop all good. Water very clean so I don't think it needs a power flush or anything, I didn't bleed
much water - just lots of air - so the inhibitor should only be slightly watered down.
Fox-
Water very clean so I don't think it needs a power flush or anything
A powerflush might still be a good thing. The difference is that the usual force of the water in the normal CH circuit isn't enough to move the sediment/crap that piles up in the middle of the radiator.
Random google link: https://hynesplumbingandheating.com/central-heating-power-flush/
A more thorough (but more expensive) approach is removing each of the radiators and flushing them individually whilst also turning them round/over so that the sediment is disturbed.
Greenbank
Just open the balance valve a bit and see what happens. Turn it back a bit if it's too much.
I think they can get blocked up if only cracked open during initial balancing. We've had a couple like that!