water not heating up on redrings expressions 500?

Hi, My Redring shower has stopped heating up on normal setting. Normally when the shower is switched on after a few seconds you would normally here a small click, the second LED light turns on and the water heats. Now only the first and third light is on, the fourth light overheat is not lit as before. The thermostat has to be turned up high (setting 5 or 6) before the water temp is acceptable, but this reduces the flow, where before the thermostat only had to be set at 2 or 3 to have a warm shower.