I think my cleaner must have clipped the TRV4 valve with the hoover or something. I went into our spare bedroom and found it very hot and noticed that the valve top was leaning back against the wall. The white plastic of the head is smashed off at the level of the metal ring. Can I replace the head without draining down and if so how do I do it? Do I have to turn off the lockshield valve, or does it involve draining down the system, which I wouldn't attempt myself. Or do I need a plumber with an ice bag to freeze the pipe?