What sort of idiot fits a bathroom without any isolation valves on the supply at all? Or any access to, for example, the cistern?
And why, when Wiesław (who is no idiot) was doing all that work---we fitted a gas boiler and replumbed the entire flat, from the main inwards, except for that bathroom---did he not install isolators on the bits of pipe you can get at?
I'm going to have turn off the boiler and the main, and swap out the shower mixer valve on a single kettle's worth of tea. Wish me luck, boys, I'm going in.