Ravenheat Combi Boiler problems?

I have a Ravenheat CSI 120(T) low nox boiler installed 29 October 2010.
Since then I have had four visits from Ravenheat engineers for a catalogue of issues and the boiler is still not operating correctly, so I am hoping someone out there may have experienced something similar.
The heating temperature is set at 66deg, the boiler fires up and continues to be fired up long after the temperature has been reached. The only way to stop the burners from firing is to either to turn the hot water on and off, or to turn down the heating temp dial.
The engineers have changed the control board, the potentiometers, and the thermostats and it still continues to run.
Ideas please