Glow Worm Compact 80e hot water problem - can you help?

Hi,

I can only get hot water from my Glow-worm Compact 80e combination boiler if I turn the water on full-blast. Sometimes I actually have to turn 2 taps on (basin and bath) before it even gets warm. Once warming I can usually turn off the second tap. However, the water flow is so fast it barely gets warm.
If I then gradually turn the tap off, the water does get warmer, but never hot, and well before I have turned it down to a "normal" flow it goes cold.
British Gas have had a look at it twice and have replaced a diverter valve(?) and then the engineer said that was all he could do except for flushing out my system which is chargeable over and above my service contract and would cost me £600!!! I'm flabbergasted that he can't do anything else, but I'm in no way qualified to question his diagnosis. Does anyone know if thet sounds reasonable or am I being taken for a ride?
Added to that, there's a 15mm pipe that sticks out through my wall which constantly drips when the central heating is on, and as a result my system loses pressure and needs topping up at least every other day. The BG engineer has replaced something there also but it hasn't made the slightest difference.
If anyone has any ideas of things I could suggest to him on his next visit, I would be very grateful to hear them.
Are my 2 problems related, or have I actually got 2 different problems with my boiler?
Thanks in advance to anyone who replies.