Monopolies and Mergers Commission (1991). "10"(PDF). London Underground Limited: A report on passenger and other services supplied by the company. Archived from the original(PDF) on 19 January 2012.
Monopolies and Mergers Commission (1991). "10"(PDF). London Underground Limited: A report on passenger and other services supplied by the company. Archived from the original(PDF) on 19 January 2012.