The battery is usually a 12volt yaesu type, long with lugs on each end. Like this one:
http://cpc.farnell.com/yuasa/2-1-12/battery-lead-acid-12v-2-1ah/dp/BT01940
A bit of research online will bring up the exact type.
You will need the 'engineer' code to enter to stop the alarm sounding when you lift the front panel on the control unit.
The batteries usually last around 5 to 7 years.
Get the type of battery bought, find the engineer code, grab the screwdriver set and you're good to go !