I think I may have the answer, When the heating is on and hot, is the pressure gauge touching the red? If so, the problem is your expansion vessel is depleted. To explain, when the pressure is too high, the boiler will not go to high gas, Your themostatic shower is reducing the flow to try and get hot water for you.
Get yourr engineer to check this, if he i unaware of this 'fault' , it is only the latter software versions of the PCB that do this.