Whitewater Aspen Thermostatic Shower - Control Knob Problem?

I have a recessed Whitewater Aspen Thermostatic Mixer Shower and the temperature has never been hot. My first port of call is to try and increase the maximum temperature setting. Only problem is that I can't seem to loosen the control knob by slackening the side screw. I understand that this might be a left hand screw so have been unscrewing this with an allen key in a clockwise direction and nothing is happening. Have also tried in the other direction and still no joy. Any thoughts on why I cannot get the control knob loosened off?