Thank you all for your comments. Yes I have used a 2.3aH battery. The alarm is actually the Accenta G3 Mini so I believe this is the correct battery. It was a like for like swap and the new battery fits in the unit. By putting the unit in engineering mode, there was no tamper alarm triggered.
The new battery had an off-load voltage of 12.7V so was charged on supply (obviously a good thing for lead sealed acid batteries). I haven't checked the old one. However, on swapping the battery the alarm no longer triggers when there is a power cut.
With regard to the charging circuit, only time will tell.....
