Who was the Deputy Prime minister when Gordon Brown was Prime minister?
At the time, the office was not in use. Peter Mandelson was the de facto Deputy.
Deputy Prime Minister: The deputy prime minister is a post that has been used at various times through British history. It is a post that is created at the discretion of the incumbent prime minister. At time of writing (2012) Nick Clegg is the deputy prime minister The post of Deputy Prime Minister doesn't currently exist.