Shower cartridge?

My shower has gone luke warm (at best) and is driving us nuts.there is no problem with the boiler...Its a combi fed integrated shower with no name or markings on it (annoyingly) I have taken it apart and removed the cartridge,I wondered if a build up of limescale might be blocking the cartridge filter...but no. the cartridge looks ok,the filters look ok yet when all back together hardly any warm water comes out,the hot pipe is hot as it enters the catridge housing but it aint comin out of it hot....where the temp control knob fits to the housing control knob is all worn down but i turned it as far as it would go with pliers so its on the highest temp and its still luke warm ( I read you can reset this by removing the circlip??..is this so? and how?
I cant replace the cartridge as I dont know what make it is, what goes wrong with them to make them broken?.. Appreciate any advice, Fraser