Hi Harry
I had exactly the same problem this morning.
Undo the 2 screws at the bottom of the unit and lift unit up, off its wall mounting (careful as the wires behind are live)
Turn the unit round - and undo the 4 screws on the back (again be careful as the buttons may fall out of their holes!). On the back of the circuit board is a round, 3v lithium battery. Unfortunately, its kind of pinned to its connectors so you'll have to use a pair of pliers to prise it out.
The reverse the procedure, making sure the battery is well connected to its terminals.
Good luck
ali
alicoates
April 2007