[stuart bowes]

I don't know if anyone is aware enough of the particular workings of the alarm PCB which is usually the bit that ends up kaput due to battery leakage

I don't know either but perhaps it's possible that the pcb is involved with the logic of alarm conditions i.e the movement sensor, window breaks, etc, whereas the red triangle simply triggers a relay which sends 12v straight to the siren.. I'm just spitballing here of course, someone may come along with the wiring diagram and confirm or deny this

If the siren isn't triggering in normal alarm conditions and you have the ubiquitous alarm service message then I would say its time to replace the siren module which includes the pcb. it's a couple hundred quid but I thought it was worth it when I did mine (although I found a NOS on ebay for 170 iirc)