Hi Steve
It sounds like you need a new diverter valve diaphram, had exactly the same problem with mine, the part itself is not expensive but unless you are very good at DIY I would recommend getting a plumber in to do it. My guy did it about 40 mins, here's a link if you're brave enough http://www.newtonnet.co.uk/house/boiler/res/blank.html
Pete
April 2009