I've run across this on e bay, see what you think.
http://cgi.ebay.co.uk/WIREFREE-SILENT-ALARM-SYSTEM-CAR-HOUSE-CARAVAN-BOAT_W0QQitemZ5869760056QQcategoryZ60839QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.co.uk/WIREFREE-SILENT-ALARM-SYSTEM-CAR-HOUSE-CARAVAN-BOAT_W0QQitemZ5869760056QQcategoryZ60839QQssPageNameZWDVWQQrdZ1QQcmdZViewItem