Interesting news story on the beeb this evening - a guy made the 6 o'clock local headlines because of exactly this problem - BT spokesperson said it is their policy to lower the speed of heavy users, particularly at peak times ( but also at other times ) to allow all of their customers to benefit all of the time ( or something like that).
BBC NEWS | England | West Midlands | Viewer unhappy with internet
This guy was tied in to a 2 year contract - maybe he ( and others ) might be able to get out of it if BT aren't providing the service that is being paid for ?