Alarm battery problem

The alarm in my apartment went off while I was away. From the readout it looks like the battery power was low - could this have set off the alarm? I don't think there was a power cut. The LED for low battery came on a few times in the few months before this but the alarm worked fine and the error LED has cleared. Do I need to replace the battery and can anyone recommend someone for this (Dublin city centre)? How much should this cost? The alarm is an Aritech and is about 8 years old. I though the alarm should go off after 20 mins - is this right?
Thanks.

The engineers log should list what triggered the alarm. Maplin Electronics on Jervis St. sell a range of suitable lead-acid batteries, just take not of the voltage (almost certainly 12V) and AH rating.

Recent Aritech turn off after 20 minutes, not sure about models from that era though.
Leo