I assume its one of the bar type mixers.
If so then pull off the front case. Pull off the two control knobs. Turn off the water supply to the shower. Undue the ribbed threaded ring at the hot (left) end. Carefully partially pull out the on/off shaft until you will see an 'o' ring (note: its not a washer). Ensure you don't twist the shaft round at any time when partially withdrawn. Take off the 'o' ring and take it to a plumbers merchants to obtain a new one of the same size. Fit the new 'o' ring the push the on/off shaft back in. Screw the ribbed threaded ring back on. Now turn the water supply back on and test for leaks before putting the case on. If no leaks then push the knobs on and finally the case front. You've now got a working shower without leaks again.
Stephen, December 2007
