Originally Posted by clever chap on the interweb

There is a self test for the alarm but to tell the truth if its intermittent you can bet your bottom dollar that as you test each point of the doors, hatch and bonnet they will all signal that they work correctly.

You can put the alarm into test mode by switching ignition on and at the same time holding the heated rear screen button. Then release the screen button and you should get a "chirp". Then open each door, bonnet, hatch and if they are ok you will get a "chirp" sound for each item (door/bonnet/hatch) you open.

More than likely the problem will lie with the bonnet switch, although another likely culprit is the hatch switch (which I will be doing on mine tomorrow as my alarm is also knackered). The bonnet switch is easily rectified by removing the switch and putting a piece of wire (bridging) between the points...effectively telling the car that the bonnet is closed, however bear in mind that when you do this you effectively stop any further alarm on the bonnet even if it is forcibly opened whilst remainder of car is alarmed. Also bear in mind that the bonnet switch sits behind the "airbag crash sensor"....be careful you dont disturb the wrong one otherwise you risk setting the airbag off.