[the radman]

that looks like a thermostat housing,more plastic junk,what happened to the good old days when things were made from metal

 

[the radman]

with a bit of luck it might just be the gasket where it bolts to the head, but theres no way of knowing until its removed ,they usually have a rubber gasket type affair

 

[the radman]

plastic stat housings have a tendancy to warp so probably a good idea that you replaced the whole thing,piece of mind etc