how can I mend my SIRRUS TS1503 shower?

The control knob that switches the shower on and off, has been sticky lately, the only way I could turn the water off this morning was to remove the disc and turn the screw clockwise.(and that was just a guess) Now, the knob just turns and turns.
Is this something I can repair myself, I replaced my kitchen taps last year, which had a similar problem.