unsure re the screwfix question, but a honeywell gen4 optima is a honeywell gen4 optima etc.
id use
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&cPath=36_62&products_id=1439
dialler
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=1425
additional PIR's
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=1007
Flush COntact
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=180