Many thanks for all your replies. It was of course just reconnecting one radiator. It occurred to me this morning when I realised the water had nowhere to go unless it could return.
My thanks again though, especially ‘foxhole’. Haven’t appreciated having a hot shower as much in a long time.
Have disconnected all radiators today to replaster and paint over the summer.
After making sure all the valves were fully closed, I went to repressurise the boiler so I could have hot water (I’ve repressurised many times with no problems).
For some reason it will not do it. Nothing wrong...