If you are certain that the knob is operatingthe switches correctly, then the fault has to be a faulty switch that is stuckin the on position, or a faulty / sticking solenoid assembly http://www.showerdoc.com/shower-spares/triton/TRITO-P12120800-Triton-Stabiliser-Valve-And-Solenoid-Assembly-P12120800
Trythe check below
If when you switch off the shower at the main isolatorand the shower keeps running then you have a faulty solenoid.
When you switch off the shower at the mainisolator and the water stops running, then you have a faulty on/off switchor a faulty PCB or a micro switch that is sticking on.