disconnect water supply to shower?

Hello,
I have a bad dripping shower it is only 3 years old, and suspect hard water has furred up the Cartridge part of the shower controls.
My problem is i cannot turn off the hot/cold water supply to the shower because i do not have a header tank in the loft, so i cannot find any isolation valves anywhere, I do have a Combi- boiler upstairs but still cannot find how to turn supply off.
I hope its a simple solution , because i am getting all sorts of stick from the misses about it! any advise would be gratefully recieved.
Regards Paul.