You can add a shut off valve to the incoming pipes (about £3.00 at B&Q) you will need a pipe cutter and some diy knowledge. You can also try (WITH THE MAINS SWITCHED OFF AT YOUR SHOWER ISOLATOR OR SWITCH BOX) unscrew the front cover, you may need to pull the shower knobs off, use a pliers to turn the shower off as the knob might just be broken
Bob, January 2010