The rfx is an external device so probably not ideal.
What you need is a detector and a stand alone reciever.
ie
Reciever
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=2126
PIR
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=1697
The only manufacturer that makes a wireless Dualtec is honeywell
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=2162
But they dont currently make a standalone reciever, so if you wanted to use this you would need a panel to recieve it.