Blair and his Chancellor of the Exchequer have an unusual relationship. When Blair ran for the party leadership, Brown agreed not to run and to support Tony. In exchange, Tony promised to make Brown Chancellor (#2 in power in the Cabinet), to give him extraordinary powers and eventually to retire and pass the leadership over to Brown.
The conventional wisdom is that if Labour wins convincingly Blair will serve most of the term and then turn it over to Brown. But if the win is narrow or there is a loss that Blair would turn it over to Brown soon.
Remember, this is a country where people talk about the Constitution, but nothing is actually written down. :-)
in other words in Parliamentary democracies you can mutiny against your own leader any time really, you don't have to wait until a convention every four years.....
Martin in Canada forced Chretien to retire sooner than he wanted to......in other words we get PMs we the people didn't necessarily vote for...we have to wait until the next national election to affirm or reject that new PM