How can I fix my Gardtec 360 Home Alarm system?

My Gardtec 360 alarm system (fitted with a Eurosec 12volt battery,type ES-1220,2 amp hr.,which has been in for 9/ 10 years) is'nt working. The display screen is showing En, instead of Un for the unset mode.On keying in the code, the system does go through the various modes & on set(Se) mode the system fails to set up when pressing the ' Yes' key.
The term En is not shown in my user manual.
Do I need to fit a new 12volt rechargeable battery ?