Nexus 8 alarm system - constant bell tamper fault?

My neighbour's home was burgled and the Nexus 8 alarm, which I fitted years ago, activated. Burglars smashed the external bell unit and cut the cable but control panel situated in the attic continued to sound so they left empty handed. However having replaced the damaged Nexus bell box with a propriatory HKC SABB external bell box / siren and reprogramming using Engineers Code handbook - BT continued to display. I then bought a used but tested control panel motherboard (in case the fault had damaged the tamper circuitry) no change. Cable replaced and continuity tested. At wits end... can you assist. Many thanks Dennis (retired telecommunications technician).