I had the same problem a month ago. I bought a replacement Diverter Valve off the internet and replaced it myself. I would only attempt it if you are good with mechanic / engineering as it was a bit tricky. Look for my other post 'Central Heating but no Hot water' which includes a step by step guide on how I fixed it. DV will cost around £100 but i have seen them cheaper on ebay. model 01-1289.
Nick.
April 2006