Re: alarm siren
The alarm control unit including the siren is located under the Near Side front wheel arch. It is towards the passenger door end of the wheel arch and is also located above the black plastic housing.
You will probably need to whip the wheel off to get to the covering, then remove about 6 phillips screws to be able to take this covering off or move it out of the way a bit. Do not forget the screw that is located toward the front end of the 'sill' area (you will probably feel a hole under there where you will be able to see a screwhead). Also take care when removing the plastic cover to not bend the metal lip on the inner edge of the wheel arch.
You should then be able to see the alarm, the easiest way to get this out is to undo two 10 mm bolts on either side of the metal housing that holds the alarm (NOT the c13 mm bolts that hold the housing to the car). The alarm control unit should then be able to be slid out towards the passenger door and you can have a play with it.
Hope this helps.
Alterantively it will be probably have to got to a main dealer so they can plug their gizmo into it to see whats wrong.