Well, not actually Pierre de Fermat on Radio 4: venerable as the BBC is, its archives don’t quite go back as far as that. (A pity: his Desert Island Discs might have been quite interesting.) But here’s a link to last year’s In Our Time episode discussing the problem and its context — still available to listen to online.
Warning: contains Marcus du Sautoy, but those with a du Sautoy allergy will find that he’s been diluted with a generous dose of other contributors. May also contain traces of Melvyn Bragg.