Sounds like the pressure relief valve has failed probably because of a blockage in shower head or kinked shower hose. Clean the head, check the hose and replace the PRV - they cost about £10.
Once the valve fails it has to be replaced as the membrane inside has split and cold water will trickle/run out of the bottom of the shower when it is being used.
It's important to check that there is no longer a blockage in the outlet.as the replacement valve will blow again.
I think the part number is 83301330 and is available form http://www.showerdoc.co.uk/shower-spares/triton/bewitch.html
Not sure how easy it is to fit. On my Triton it is very easy with no plumbing skills needed
Brian Evans, January 2010
