Hello all
I am pleased to report that the procedure was successful, with no unexpected siren alarms or deafenings (repeated here in full for the sake of completeness):
1. Remove cover from external bell box - at this point the external siren should not start sounding, because alarm is not armed on control panel inside.
2. Disconnect battery inside external sounder - the external siren will still not start sounding.
3. Remove front from control panel: panel tamper spring will release and internal alarm will sound, so reset alarm with keypad - internal alarm is now silent.
4. Disconnect battery inside control panel - neither external or internal alarms will start sounding.
5. Remove fuse for control panel circuit to disconnect from mains - neither external or internal alarms will start sounding.
6. Unclip circuit board, so it can pushed out of the way to reveal all wall screws.
7. Unscrew control panel from wall.
8. Replace circuit board.
9. Reconnect battery inside control panel.
10. Replace front to control panel.
11. Replace fuse for control panel circuit.
12. Reset alarm on keypad if necessary.
13. Reconnect battery inside external bell box.
14. Replace external bell box cover.
Actually I didn't reconnect the battery inside the external bell box yet (#13), because I still need to mess around moving a couple of sensors inside the house, but I'm assuming this doesn't do any harm to the external sounder, just disables it?
Many thanks to all those useful suggestions above.
Cheers