My Aqualisa "On/off control knob" spins freely?

The teeth appear to have been stripped off. I replaced this six months ago beacuse of the same problem. Rather than replace this part every six months, I am looking for a better solution.


Aquavalve 605 thermostatic and 405 manual mixer valve
Part 1
http://www.aqualisa.co.uk/parts-store/Controls/Aquavalve-605-thermostatic-and-405-manual-mixer-valve/