Yep, turn power off or remove the fuse from the fused spur before you open the control panel, replace with the same amp hour rated battery.
Cover back on mains on, your user code input should reset it and restore order.
Been in the trade for many moons now, and I have the octagon on cd, can't expect me to remember everything related to your panel.
Good luck power off first and be safe :)
e manno
September 2012