Theres an 'o' ring that fails just inside the on/off shaft. Undo the plastic nut and partially withdraw the shaft taking care not to turn it. You then have access to the 'o' ring and this can be replaced, coated with silicone grease and then reassembled. You have a working shower for under £1 instead of £130 for a new cartridge.
Stephen
December 2007
I had the same problem with mine.There is a plastic nut that holds on/off mechanism into the cartridge. You could try tightening this. Mine turned out to have a crack in it, and as you cannot buy these anywhere, I will have to buy a whole new cartridge!
Richard
June 2007
sounds like a seal has gone in the thermostic cartridge.