Do the radiators get hot when you turn on a hot tap?.
You may need to fit service kit t0019. This is for the diverter valve that directs hot water round the radiators or through the domestic hot water heat exchanger. Perhaps fulcrum arm movement is too small due to a failed DHW demand diaphragm (common problem), or perhaps diverter valve bobbin&washer &'o'ring need replacing (included in the t0019 kit). Not a huge job, if you are reasonably practical, have the user manual and time. When you turn on the hot tap you should see the fulcrum arm move, and see the microswitch operate that tells the boiler you need hot water.
Ian
January 2007