I honestly don't know if it will be useless mate to be honest. I have a 300Tdi, so haven't had personal experience, I've just read a fair bit on Landy Zone about people having bother with the cheap ones. It might not be all of them though!
Did they break in to it? Don't fall for the faulty alarm trick where they come back several nights in a row and set the alarm off, by shaking it or something, until you get sick thinking it's a fault and don't put the alarm on. Next time they come back the alarm doesn't go off and they're in. It's an old trick so modern alarms might be to good to be fooled.