Whale prestige cold water tap not switching off

sootyfaestoney

I am new to the site, and have tried all tinternet to find a solution.The cold water tap on my motor home will not switch off when pump is running, I have to isolate the pump to stop water flow. The tap control seems to rotate freely, I presume that something
is broken. How do I remove the control to see if it is indeed broken. Do I have to remove the complete unit as I cannot see a grub screw on the tap.any advice would be better than I have just now. Thank you in advance.