Shower fitting not fit for purpose - what are our rights?

We purchased a bar-style shower fitting from one of the largest bathroom manufacturers, via a local independent bathroom shower in November 2009, which has a 5-year warranty against defects. We live in a hard water area and after a couple of months we started to have problems adjusting the temperature of the shower. After getting a plumber in they advised it was an issue with the way bar showers work as the cartridge which controls mixing of hot and cold water gets scaled up. I wrote to customer services and they sent a replacement cartridge. We are now at the stage where we are swapping the original and new cartridges every week or which entails turning off the water, taking out the cartridge and soaking in descaler before putting back in. I noticed on other manufacturer's website they state that for the bar-style shower you may need a descaler in the system, hence not recommended for hard water areas. There is nothing on this manufacturer's site. I believe the shower is not fit for purpose and shouldn't be sold to customer's in hard water areas. I would like the manufacturer to replace the shower with a thermostatic control. I would like it to either be the same size fittings, or for them to also pay for any retiling that will be needed. Given that this is now 18 months old, what are my rights re. Sale of Goods Act or any other legislation that might give us some protection.