Mira mixer shower won't stop running

Hi, you should have isolation valves for the shower, that said the pipe work may be inaccessible so you may well not have!..(not helpful I know but it will save you looking removing tiles etc ..) if you have a shower over the bath pop the panel and have a look for 2 x 15mm isolation valves coming off the tap feed to the shower feed.if it feeds from the roof space above the shower you could also check there . Mira sell a rebuild kit for the element, although personally I don't like fitting them they always seem to not last long after rebuild..

Sounds like a new cartridge is needed in the body of the mixer unit. As for isolation all depends on how it was installed, for works you can always turn of the mains to the house, usually a stop cock located under kitchen sink.