Hi Jude
is you alarm supply on the same circuit as the fridge or freezer?
 A neighbours alarm was doing exactly that, same time every day,they asked me to have a look, I found that the installer had taken a live spur from the same line that the fridge was on.
when the thermostat kicked in on the fridge a slight power surge activated the alarm. i moved the supply to a dedicated supply ,problem solved.
                
 jp
                
 January 2007