Stephen Sackur speaks to Martin Amis, an author who was pigeon-holed early in his career as the ‘enfant terrible’ of the British literary world. Four decades on, he remains one of the most successful and closely scrutinised novelists of his generation. His books are filled with greed, lust, addiction and ignorance, and yet, he suggests he writes in a celebratory spirit. So, what exactly is he celebrating?