logic 4 alarm system how to fix a problem?

Hi, the logic 4 key operated alarm has been installed in my house for years, it was already fitted when we moved in (1999), it's never been a problem until now, it has started sounding randomly for 3-5 seconds every now and again with no pattern, I have tried re-setting it with the key and even if I leave it in TEST it will still randomly sound??? I have tried removing the battery in the control panel and turning the mains supply off to the alarm in the hope it will reset, no joy and if I do this the alarm will sound continuously, there must be a battery in the bell outside? can anyone help please?

Gareth
April 2013