How do I fix my ADT Galaxy 8/16 alarm system?

Hello,
I inherited an ADT Galaxy 8/16 alarm system with Series 6000 control keypad, when I moved here 2 years ago.
The green light on the 'Series 6000' control panel has been flashing for a day or so.
This afternoon, the panel started sounding a steady tone for a few minutes, and the indicator light was steady green. I tried entering the security number, but the display was not working, and there was no response to the number. Soon after this, the alarm went off. I tried re-entering the number but this did not work - panel still dead.
I removed the cover from the electronics box and checked the fuse - it's OK. Disconnected the battery, reconnected it, and the alarm stopped for a few seconds, then started again. Tried switching the house mains off while the battery is disconnected, but the alarm still sounds.
After 20 minutes, the alarm stopped.
I rand the ADT Help Desk, but they weren't feeling helpful today.
I'm out of work and cannot afford to pay nearly £100 for an engineer call out. I am a competent electronics engineer, and I just need to know how to disable or fix it so it either works properly, or so it doesn't go off in the middle of the night. Short of borrowing a ladder and climbing up to the alarm, hammer in hand, can anyone help with advice please?