Suzanne - if you want a cheap tour bus, route 11 goes past a lot of the major sights (it used to anyway!).
nit-tastic tip
ty - should have saved that one for the nit thread maybe!
Here's a link to the route by the way - takes in St Paul's Cathedral, Trafalgar Square, and Westminster (big ben, houses of parliament, westminster abbey)
http://www.tfl.gov.uk/tfl/gettingaround/maps/buses/?route=11