Goodbye Downing Street

Once you've got the top job, how do you know when the time is right to stand aside? Very few Prime Ministers leave Number 10 voluntarily at a time of their own choosing -- most are ejected either by their party, like Margaret Thatcher, or by the voters. Tony Blair has taken the unprecedented stop of announcing his departure in advance -- hoping perhaps that he can avoid the messy and painful departures that have so often marred the final days in office of prime ministers. Susan Hulme looks at some of the brutal lessons of history for Tony Blair.