how to fix Shower faucet
 

We have a Rohl R2012C pressure Balance rough diverter faucet for our shower. We also have an on demand water heater. We also have hard water (i.e. small stones) The problem is when showering we can't regulate the hot water. Other faucets can regulate OK. I think the screens in the valve have trapped particles. I can't get the filter stop valve open to clean filter. I don't want to loose parts behind wall.
any help would be great. I have the spare parts manual but no help. Thanks