My son has the same car, same engine and had the same problem.

You either need to replace the whole radiator or, assuming it's a slight leak, do as we did and put some Radweld in. Some on here wouldn't touch the stuff because basically it's a bodge, but I consider it a useful aid to bangernomics.

If you've taken it off and found a leaking crack, I would try coating it inside and out with a loose paste made from fibreglass resin and filler powder.

I might try it, but I wouldn't put much money on it withstanding regular cycling in hot slightly corrosive solution. Even the bottom of a radiator can get quite warm.