https://986forum.com/forums/performance-technical-chat/35081-battery-alarm-problem.html
Do you have option M534?
"Vehicles for the UK and Belgium are equipped with an alarm siren and a tilt sensor in addition to the standard alarm system. The M 590 (locking system for the bonnet controls) is also fitted as standard."
Check
https://www.porscheclubgb.com/forum/threads/alarm-developed-a-life-of-its-own.143723/ also
I would try to get a new battery from
https://www.hamilton-palmer.co.uk/. Alternatively, I would try to disassemble the old one first to see the specification of battery.