The plumber changed our 3 way valve but everything still seemed stiff and not quite right. He reckoned the 3 way metal pipe behind it was responsible. We paid him for changing the valve and his 2 hours of checking etc.
The next day he phoned to say had we kept the box from the replacement valve as he had now got hold of one that came with the 3 way metal pipe attached. He could change the new one for that version and all would be well plus give us a full rebate for the new one just fitted. He could then get a return on said valve.
He came yesterday afternoon, partially drained the system and changed the valve and pipe. Another bit of testing and all was well. When he and DB inspected the metal pipe, it was corroded and pitted from limescale hence not opening and closing correctly.
Not too expensive in the end and a good repairman, willing to sort things out and not con us. We shall use him again for future jobs when necessary.