Leaking car radiator

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I have a 4 year old Fiat Idea which has developed a small leak under the radiator, somewhere between the plastic expansion and the metal radiator.
Rather than go for an expensive repair, Fiat are quoting upwards of £90 just to look at it, is it safe to use the 'Stop Leaks' liquids available in most car accessory stores? And do they actually work?