Your alarm system has 3 power sources:
Endstation: Mains supply and Battery (12v 2.1AH Lead Acid).
SAS (EXTERNAL - Self Activating Sounder): 6v NiCad (Probably)
So would guess that when their is a powercut, your system activates because the battery inside the endstation has depleted and has insuficiant voltage/current to power your alarm panel and the system dies. The SAS is designed to activate when its main source of power is removed (feed from the endstation), when this happens the SAS will run off its own battery until either a cut-off timer kicks in (15-20 mins in accordance to comply with the noise pollution act) or until the battery depletes (old SAS's did not have a timer circuit).
The best option is to replace the Internal battery, I would suggest a Yuasa 12v 1.2Ah. Or if you want a cheaper solution, climb up a ladder open the SAS and disconnect the battery. Please note that alarm systems have tamper circuits that when tripped will create an alarm condition (i.e. opening the endstation to change the battery or opening the external SAS).
Hope this helps
PR Shields, May 2009