Thanks for the help, it's difficult to see how the battery is fixed to the casing, when standing on a ladder 5 metres above the ground - 1 hand for me and the other for working. I'll use a screwdriver to prise it off since it appears to be simply stuck on and no help notice. What is worse, that not being left handed, the right hand is for holding on and the useless mitt is the left hand for working on. Why there is no space at the left hand side for ladder access ,is probably because it was installed by a left handed installer. I now have replacements for both 9 year old batteries. These alarm boxes needn't be mounted so high, I think that 3m is high enough.
Jaymack