This is a common problem, it is usually caused by a build up on the spindle causing the rubber seal to wear. you can replace the brass unit by turning the water off (black tap on 15mm pipe under boiler)and opening all the hot taps then undo the 4 screws and remove. Replace with new unit (around £20). Check that the water has not entered the microswitch, if in doubt replace this as well.
Ian
May 2006