Rarely does the death of a long retired politician prompt a genuinely worldwide reaction, but Margaret Thatcher was one of a kind. Britain's first female prime minister transformed her own country, and provided the world with a model of market economics and conviction politics, which was inspirational to some, repellent to others. Stephen Sackur talks to a man who played a key role in the creation of Thatcherism. Maurice, now Lord Saatchi, was the advertising guru who helped define, and sell, what she stood for. How enduring is the Thatcher legacy?