Fiesta with Endura-E engine leaking coolant - what's this part called?

you want a 'thermostat housing gasket', from a ford place will get a better gasket than the average car shop, replacement should be easy, undo the bolts, clean both surfaces, replace the gasket and bolt back, I would usually recommend using a gasket glue (of a suitable type) as well, two bolt housings easily distort and the glue will take up a fair bit of distortion. catch the spilt coolant and pour it back or just add antifreeze to top up.

distortion.

Thanks for that. I've just bought a new gasket from a Ford dealer for 41 pence. They looked aghast when I asked for gasket glue, but I'll probably get some from Halfords just in case before I get the socket set out.

Ideally you would not use any sealant, but reality is that when things get old or overtightened and are a poor design in the beginning (only two bolts to hold a circular seal is poor design, but slightly cheaper than three bolts) then a bit of glue will keep it sealed for longer. A sealant like hylomar or the correct loctite one for the job will be fine.

Stuff em. The Japanese tend to use it everywhere and produce leak free long lasting engines.

I strongly recommend new bolts as well. Cooling system bolts are often corroded and much more likely to break if re-used.