Hello Claire,
The battery you require is a 12volt 2.2Ah (£10 approx).
You can change this yourself by turning off the mains power to the main panel and disconnect the battery .
Once disconnected the outside bellbox may go off ,
don't worry just put in your new battery and switch on the mains power,screw the front back on, the alarm should make an audible warning, enter your code and press reset ,that should be it (or something along those lines).
Your battery should be fully charged by the alarm panel in about 24 hours once installed!
Hope this helps
p.s. let me know if it works
(sorry for not answering any quicker but i have just got out of hospital)
john from stoke, July 2006