Stephen Sackur interviews Sir Ranulph Fiennes. He is an extremist of a very special kind - nothing to do with his political views - but recognition of a lifetime spent embracing physical challenges at the extreme limit of human endurance. He has taken on - and conquered - the polar ice, the world’s highest peaks and the most gruelling deserts. He's been described as one of the world’s greatest living explorers. So what's the motivation for this life of extreme adventure?