When a headhunter phoned Tidjane Thiam, then working in Paris, to discuss a job with Britain's biggest insurer, Aviva, he laid down a condition. "I said, 'well, I am very happy to interview. But frankly, you need to tell them that you found someone who is black, African, Francophone and 6ft 4'," he laughs, his mouth shut tightly and his broad shoulders shaking at the recollection.