triton combi hp shower?

please could anyone help me, my wife is going crazy without our shower....we have a triton combi hp shower but the water is cold, could anyone tell me what the problem is, it goes barely warm but thats all....could it be the thermostat or another part and if so are they easy to change as i am no expert and are they expensive? I dont really want to change the shower as it will effect the tiles in the bathroom so if possible an easy solution of fixing the shower would be great, thanks