On 27 Jan 2020 07:23:19 GMT, Tim+ wrote:

would like to leave the new owners the option to reconnect it
easily
if they so wish.

As others have said, you have a dead back up battery.

To decommission it properly you really need the service code, not just
the regular activate/deactivate code. This allows you to open the box,
change sensors etc without activating the tamper circuits.

Be warned that the external sounder will probably have its own battery
so if you don t have the service code, removing the mains supply and
the backup battery will be treated as a tamper event and trigger
the sounder.

Which probably has a knackered battery as well after at least ten
years of no maintenance.

These things can very noisy for a *long* time.

Shouldn't sound for more than 20 mins. Operative word "shouldn't" and
see above. B-)

Probably best to tell neighbours you're going to do some work on the
system after the false alarm and it might sound a few times. Don't
tell 'em it's going to be decommisioned...

Get access to the external sounder(s) and some ear defenders. Open
the external sounder, it'll sound due to tamper, disconnect it's
battery. Switch off the mains power to the panel and it'll probably
fall silent. Open the panel disconnect it's battery. Leave mains
disconnected, take fuse out of SWFCU? Job done.

--
Cheers
Dave.