How to change the Alarm Siren

You need a a 10mm/12mm spanner, a ratchet ring spanner makes things a lot easier
Magnetic pick up tool in case you drop a bolt.

Open the boot panel flap on the left side and undo the knurled knobs shown below (black-bottom and white-top)
if they are tight try getting a small towel in there for grip

Withdraw the lamp towards the back of the car and disconnect its loom

You should then see the siren below.

Disconnect the wiring loom and use a 10mm spanner to undo the bolt at its rear.
If you get the revised part as shown below then ensure you get a bolt as its 12mm and different.

Fit the Siren, reconnect loom, refit the rear tail lamp and the job is done.