I have a C & K Securitech C023 203 alarm. I recently turned of the mains power in order to replace a fuse in the main fuse box. The alarm was not set at the time however turning of the mains set the alarm off, I could not disarm the alarm by entering the code. Eventually the alarm stopped ringing and the mains reconnected the alarm stayed silent. The exterior box continues to flash. I am unable to set or reset the alarm, I have tried both our code and the default. Neither the mains or fault lights are lit on the alarm panel, I think the panel has power as the buttons light up and beep when pressed. If I need to replace the battery where is this located?