Hi Tugster, We haven't yet had a false alarm with ours as the motion sensor is InfraRed so it will only respond to the heat of the body of someone inside the van - totally immune from wind etc. One of the reasons I liked this one was the fact that it has a second external sounder which will operate if the wire to it are cut and the remote key fob arm/disarm trigger.
As I said, have a look in B&Q coz they sell basic door/window alarms as well, some with keypads and some with keyfob type remotes. Another firm is Maplins (www.maplin.co.uk), if you haven't got one near you, they do mail order. At the NEC SAS security were doing a pack of six very basic door/window alarm units - sounder with integral magnetic switch for about