How to reset Itec Horizon Panel (also Aritech CP35)?

I had a power cut today with the lights flickering for about 10 mins before the power went off completely. When the power came back on the alarm started sounding (intermittently rather than continuous) and I cannot turn it off - it seems unresponsive to any button presses.
I switched off the power to it and removed the battery (which was dud anyway) and left it for an hour but as soon as I restore power to it it starts alarming intermittently (once/sec) again. I can't see any reset buttons/switches on the panel board. Any suggestions gratefully received.