Luke warm water through MIRA 88 shower?

Since I have put the central heating on for the winter, the hot water that comes through the shower is only luke warm, even though the temperature valve is on the hottest setting. We can make the shower a bit warmer if you really hold down the temperature valve or have the cold water tap on in the basin. I have a combi boiler. My dad has said something about a filter. Do I need a new shower or can this be fixed?