Gus O'Donnell has been at the heart of government in Britain for 30 years, working closely with the last four British prime ministers. He was John Major's press secretary; under Tony Blair he took on the top job in the civil service, a position he held when Gordon Brown took over. And that meant that at the last election he was the one overseeing the negotiations for Britain's first full coalition government since the Second World War. After nearly two years with David Cameron as prime minister he has now retired. And with his former role being split into three jobs - does even the man who signed his letters by his initials G.O.D - recognise he was just too powerful?