Shower Cold?

I just moved into a property and the Bristan 1901 thermostatic shower runs luke warm/cold, I believe the unit has been in for a couple of years. It runs from a combi bolier and the cold water pressure seems very high. The pipe going into the shower is very hot. Is it likely that there is just to much cold water and I need a pressure reducing valve for the cold or is it likely the thermostat in the unit has gone faulty?