Strobe light won't stop flashing?

We had a power cut one night and our 8SP 400 ACCENTA MINI G3 alarm activated. The 'tamper' light was also lit on the remote LED keypad. I reset the alarm by entering the reset code. Alarm stopped but external strobe light continued to flash and has continued to do so for the past two day and nights.
I have tripped and reset mains power CB and alarm has activated again. Is the battery flat? If so, how do I change it? Thanks.