The Prime Minister’s Office forms an integral part of the Cabinet Office. The original requirements for Departments were set out in Sir Peter Gershon’s “Releasing resources for the frontline: Independent Review of Public Sector Efficiency”, published by HM Treasury as part of SR2004. The review, including the original Cabinet Office requirement, can be found on the HM Treasury website:
http://www.hm-treasury.gov.uk/spending_review/spend_sr04/associated_documents/spending_sr04_efficiency.cfm
Copies are available in the Library of the House.