We gathered relevant information from a number of sources including:
■ an extensive examination of the Department's papers, documentation prepared by advisers, and bidders' submissions;
■ interviews with the Department's relevant officials and advisers, on how they approached the development of the PPP and managed the selection process;
■ the financial models submitted by the Airline Group in support of their bid, with the sensitivities run by them at the request of their bankers and of the Department;
■ monitoring of reports and commentary in specialist aviation industry websites and news "feeds", including public discussion boards used by NATS staff;
■ interviews with staff in the safety, economic and airspace regulatory groups of the Civil Aviation Authority's, covering the design of the PPP and the future regulation of NATS; and
■ a survey of bidders participating in the auction, supplemented in the some cases by detailed interviews.