Thanks for the help lads. Replaced battery and everything back to normal.
For anyone else that comes across this, it is fairly simple to replace the battery.
Get up to the siren
Lift up the little plastic flap covering the screw
If battery still has some juice, it will trip the tamper, else nothing will sound
You can lift the PCB up and out of the housing fairly easy if want to replace on the ground (good design Pyrnoix!)
Replace the battery in the white plastic housing
No learning procedure required
Slide the PCB housing back into the main siren housing
Note, the tamper may trip (it is the little contact relay at the front). Hold it down if bothered by the noise
Lower flap, put screw back on
Set alarm, trip and test siren sounds