FatPhil, having a ding-a-ling alarm is'nt much use. Get one that phones you and / or sends a text message. Mine tells me which sensor has triggered, including ones outside and in the genny shed and garage. If I can't get back quickly then I can phone neighbours. If you have pets then you need to choose suitable sensors. We were robbed 4 times until the alarm went in and a neighbour twice. Not been done since (4 years ago) and I'm much happier knowing that it is safe.
Friends have a 'managed' alarm where the security van turns up 4 hours after the alarm has gone off and pays $30 a month for the priviledge!