There's a little slot on the right hand of the bottom of the unit. Switch off the mains, push a screwdriver blade into this, and the whole of the front flips off, revealing the works on the back of it. The battery is a green thing in one corner on mine, which lasted 18 years before it died! Get a replacement from a good plumbers supplier. Take note of which is the + contact on the unit and battery-- on my unit the board was not marked at all!
BryanL
February 2009
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