Put some black pepper in it - about a tablespoon. The pepper absorbs the water in the radiator and swells up. The pepper will then clog up the hole in the radiator. This repair will give you enough time to get to the garage.

Or Bars Leak has also given me excellent results when I had a fin split. I cut the fun in half and rolled up both ends and threw in some Bars and then did 80mins on the F3 in Sydney.