Paul, Constant, thanks to both of you for your inputs.
From what I can fathom out all Cat 1 alarms/immobilisers have to have door boot and bonnet triggers. Additionally I think they have to have a minimum of two methods of immobilisation, either affecting the fuel, ignition or starter circuits. Most systems out there seem to have the ultrasonic interior sensors, however I believe it's the microwave sensors we need rather than the ultrasonic. Most systems appear to be modular whereby you can add additional modules to your existing system, for example if you later wanted a tilt sensor, it can be plugged in, if you want a glass smash sensor this can be plugged in and so it goes on. Some of the latest systems will send a text message to your mobile phone to let you know the alarm has been triggered. If your car is stolen, some systems allow you to send a text message to your car and it will shut it down. Other systems allow you to track the car on the web. So there are great advances in these systems out there but trying to sort out which is the best, most reliable and effective for our types of car, isn't easy.