Ravenheat hot water problem?

Hi I have a ravenheat rsf 20/20 combi boiler, and we can only seem to get hot water out of it when the heating is on and the tap is turned on slightly. We have replaced the heat exchanger and diverter valve but neither have helped. What else can the problem be? Its been like this for a month now and my patience is wering thin with so called engineers who are only out to rip you off. Does anyone have any names of good honest reliable companies who may be able to help me without ripping me off??? in the Buckinghamshire area.

Many thanks