Simples, as per title, or do I need to order from the USA?
I had to order from the USA ade.
It is very good stuff but expensive with the tax added to it.
I need a header tank and there don’t seem to be any other sensible choices 
Yep got the header tank and bought the Cosworth cap in the uk, the instructions you get with it are very good. Make sure you take your time reading them, I colour coded my hoses with tape which made it easier to follow the instructions and not get muddled up because you have to cut the original hoses to suit.
Had to go direct too Ade