Stu, get one of those Motorcycle 'rollerball' alarms mate. How they work is this: The alarm has a chamber with a metal ball bearing inside. When the bike is parked, and the stand is used, be it the centre stand or side stand, the metal ball finds its place and stops moving. The alarm is set, and as soon as someone moves the bike to take it off its stand, the ball rolls and activates the alarm. It works the same way with a car as they are 12 volt systems. Once the car is pushed, moved etc, the ball rolls and activates the alarm. To make it even more effective, Go to Maplins or somewhere similar, and get the loudest most anti social 50 million decible alarm or whatever you can get, and wire that to the Motorcycle alarm! ; ) It is impossible to move the car without activating the alarm!
As much of an inconvenience as it may be, if you dont use the car everyday, put it on axle stands as well, and even go as afar as removing a few or all of the wheels. If they want your car, they have to carry a set of wheels and a jack with them, and as soon as they jack it to remove the axle stands, the rollerball alarm will activate, so they wont get very far at all!
If all of that is too much mate, and you have a mother in law, lock the bitch in the car every night, and see how pissed she will be when a couple of sewer rats try and nick it and wake her up!! Good luck, sleep easy.....