This is definitely from the rad itself and the leak has gotten worse. Two questions:
1) I used epoxy glue to repair the first time and it worked for a a few months but was difficult to apply/get all the way through the fins of the rad. How ridiculous/sensible would it be to use silicone sealer? It has a very high temperature resistance and some can be applied in damp conditions. Just a final saying radiator thought
2) The used parts guy in Wales didn't have any and said they are rare as gold as a used part, he even purchased a used one from the USA himself a few months back. I have found a copy part on eBay "made in China" and wondering if anyone has had experience of these? They are a tempting price at £225 delivered but obviously not sure about the quality, I have experience of Chinese parts and my gut says these would likely be OK, since they are a handmade not mass produced product. Mass production Chinese stuff in my experience tends to be flimsy.