hi mate am fixing the same problem as i type.... well not quite at the same time. but i had an identical fault, turned out to be a perishe diaphragm inside the diverter valve i was quoted 200 -300 quid buy a worcester engineer. so decided to do it myself as the part was only £15.... here is a link to the site that helped me i hope it helps you.
http://www.newtonnet.co.uk/house/boiler/
Den, May 2006