For anyone else with the same problem, I've got a 70 quid chinese radiator on my Gen 1. It fitted fine and it's done a few thousand miles now and it's not leaked yet.
It is definitely not quality though. Very thin mounting lugs and seems to lack solidity. I did slap another 3 or 4 coats of paint on it before I fitted it, so who knows how long it will last.
I will be getting my old radiator re-cored by the local radiator specialist for when it goes west, and the prices aren't stratospheric, so have a word with your local guys (Serck are good and nationwide)