or just get a decent steering lock for it and remove one of the HT leads or something when leaving,they can't rob it then unless they tow it...in the old days before cars had electronics and stuff and you could break into them with a teaspoon or lollystick i used to just remove the rotor arm, or LT lead off the coil then even the most determined joyrider wouldn't get it going!!
alarms are a general pain in the ass i think and nobody takes any notice of 'em going off...
if you have a good diskloc or similar fitted then the casual car thief scumbag will move onto easier pickings.plus make sure you don't leave owt of value on show-satnav,phone,radio front still attached,etc,,i know its obvious but amazing how many folk do,majority of thieves will brick your window for this kind of stuff...