I was going to change the said part in the title, for my shower as it is looking a tad tired. As I understand it, it should just un screw ready for fitting a new one. When I try to unscrew it, it just turns and turns, with the thread pipe into it. Why would this be, could it be into push fit plumbing behind the tiles/stud wall, hence just spinning in the push fit that I cannot get at without removing tiles etc?