The radiator on my 420R developed a tiny pinhole leak in the upper/LH side corner, which apparently is a common failing point from my reading here. Car has about 3k miles on it, mostly track. I have read the theories about excessive vibration transmitted from engine and/or uneven mounting points putting stress on the radiator. I ordered a new standalone radiator from CC. They list only the combined water and oil radiators for the 420 road model:
However, you can get the water rad alone. It says its for the 420 race model only, but mounts up just fine. You just don't need the two little upper drop brackets anymore:
I put on the new rad, and copied the idea of a user here to try to reduce transmitted vibration by replacing the distal segment of the top coolant hose with the 620R top coolant hose: (https://caterhamparts.co.uk/hoses/5758-cooling-hose-top-seven-620.html). The hose isn't perfectly shaped, but it's pretty close. Side benefit is that the hose is flexible enough that I can now pull it out of the way enough to pull out the oil catch bottle to drain it, and will no longer need to suck it out.