I have just been told by www.showerdoc.com - help line 0131 538 4343 that the easier way to solve this problem is to replace the 'internal cartridge' costing £98 with 48 hour delivery. Think I will try and get a plumber to do this because he said if not fitted correctly water can drip from it. Hope this helps.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.