How can i fix a broken temperature contol knob (worcester 24 cdi)?

I have a worcester 24 cdi boiler fitted in my kitchen and have broken one of the controls knobs. There are three knobs from left to right the power control, Heating temperature and the water temperature. Last night i adjusted the heating temperature control and something inside the dial cracked. Now the dial rotates without any stopping. Do you know if the control knob can be replaced or is there likely to be damage inside?