I have a Potterton Promax 33 HE Plus combi boiler. All appeared to be working fine until a couple of days ago. Hot water only is selected on the boiler front, the boiler heating water temp is turned right down and the roomstat is turned right down. However, when I run hot water the radiators get hot. When I stop running hot water the pump overruns for short while then shuts off. The boiler is not indicating any fault condition, the pressure is fine, everything appears normal. Any thoughts?