Isolating water on concealed shower mixer to fix poor hot water flow

[JohnnyPlatt]

Morning Gents,

Hoping someone can help.

We have a concealed shower mixer in our bathroom and over the last few months, it's slowly reduced the flow of hot water to the shower.

I have read about this and understand that I need to remove the central cartridge. HOWEVER, when I finally cut away enough tile to see where the water enters the mixer, there are no valves, just two huge screws. I tried tightening them and they wouldn't budge. I then tried loosening them and water started to seep out... So I screwed them back on. London story short - I'm figuring that they aren't going to help.

Will I need to turn the water off at the mains to do this?

Also, any idea how I can find out which cartridge I need to replace this with?

Hope you can help.

John