disabling home alarm

we have an alarm which came with the house - the previous owner had let the maintenance on it lapse, but we've no engineer code for the alarm. and judging by the paperwork i've found, it was installed in 1997.

the problem we have is that the alarm started glitching a few months ago, so we stopped using it, but now the alarm box - the one with the main circuitry and battery - is getting excessively warm; the battery itself is getting warm, and there are two heatsinks on the circuitboard which get too hot to touch within ten minutes of power being restored to it.

the problem we have is that if i pull the fuse on it (i'm obviously worried about a fire hazard) the main external bell goes off within about an hour - is the only option to get up a ladder and disable it at source? i assume (especially since we don't have an engineer code) that they're designed to prevent suppressing the bell from within the main box...