Press the 'Stop' button then briefly press the 'Low' button.
The light comes on automatically after a certain period of time to remind you to clean the shower head, based on an estimate of how dirty the shower head will be after a certain period of time. It does not actually sense how dirty the shower head is.
Full instructions for the shower unit can be found here:
http://www.partsarena.co.uk/baxi/System/DATA/Dx/DS1/installation/2923/i48-2923/i48-2923.htm
Steve, July 2012