The Trafalgar shower is thermostatic. I assume the centre cross head tap is what you are trying to remove as this controls the temperature. The temperature is set by turning the knob as you know.
I'm unsure what you mean by a stop ring. Do you mean the knob is stuck on the cold side? Did the control have a button to press to take water temperature past the factory set temperature?
The first thing to do is check how freely the middle know turns and to not how far it turns. If this is reduced from before you had the problem then the stop ring could be stuck. If it still has the full range of movement the problem is the thermostatic cartridge has failed.
To get the tap head off may take more force than you would expect. It slides onto splines and these can get seized up. Once you have the central screw out from beneath the cover, check that there is no allen key as a back up to the screw. If there is unscrew that. When you've this get something like a chamois leather and cover the tap head. Next get a set of adjustable grips and clamp the tap head through the chamois. Now firmly pull and at the same time wriggle the tap head to free it off the splines. If this fails try levering the tap head if you can get a piece of metal between the cover plate and the tap head. carefully use the lever and the grips at the same time. Hope that works. When it's off remove the stop ring and see if you can get a hot shower. If so install the new stop ring and check operation. If you don't get hot water then the cartridge is at fault.
