How can I replace the temparature control knob on my Triton hp shower?

The temparature knob broke off and I have bought a new replacement set - the whole white knob. How do i replace it now? I have removed the horseshoe clips(s) and now I am stuck as the broken bit is still in there. Do I need to turn off the water from the source? Someone please help me as my husband is not amused!