 In late 2000, he was made Special Adviser to the then Secretary of State for Northern Ireland, Rt Hon Peter Mandelson MP.In 2001 Diamond bid for election to Lambeth Council, though failed to win a seat in the 2002 Lambeth Council election.From June 2001-September 2004, Diamond was a member of the Number 10 Policy Unit with particular responsibility for the government's public service reform agenda.