The filling loop looks like the image on the link
http://www.cheapboilers.com/images/product/filling-loop.jpg
These do vary, but it's a good way to start your search. Generally speaking, it's usually fitted in proximity to the boiler itself. But you're talking a Sine 18 which are rather old, and configurations for filling points differed greatly in the 80's
Carefree, June 2012
