The hose from the expansion tank plugs into a.

Yipes! You need to upgrade that plastic thermostat housing to an aluminum piece. As you can tell it becomes brittle and pieces snap off over time. If you are looking for a replacement for the part that snapped off it can't be done. You need a whole new housing, do not JB weld, a whole new aluminum piece is $60-70 and that will ensure no leaks from the thermostat or the base of the housing in the future. If you are talking about the actual rail that runs to the radiator expansion tank I believe that is the radiator tank rail. Check out jaguarforums. Com, lots of info there on how to replace the thermostat housing.