To expand on what mav said but in more laymans terms, its worth trying it at times of the day... You buy in broadband with a "contention ratio", usually about 15-25:1, what that means is the bandwidth (size of the pipe connecting your house to the internet) they sell you is actually shared between 15-25 other people on your local exchange. So if you try it at say 6pm maybe there ARE 20 other people trying to use the net at that time of day, so the connection bandwidth has to be shared between everyone and you only get a dribble down the pipe. Lines that have no contention cost a absolute fortune, as this is the only way they can deliver cheap broadband as the real cost is the bandwidth.
Also your uplink speed will be crap compared to the downlink. They offer the same service style as ADSL, which is asynchronus meaning that the size of the pipe connecting you to the internet is bigger in one direction than the other. Its like that for a reason, most of your bandwidth requirements is down, your web browser might go to a webserver and say "give me this web page", to which the webserver responds with a webpage including images etc, your request might be a few bytes long, yet the response a few hundred thousand bytes. So the provider isn't going to fork out for huge uplink speeds when most of the time it will sit unused and just cost them money to buy in...
The other thing that strikes me is have you scanned your computer for spyware etc recently? when I last got a look at NTL's cable network it was open, there was no filtering or nat or anything on the modems, and poking round the local subnet we could happily browse the contents of everyone elses hard disks etc. In those situations its possible for a script kiddie to have installed a backdoor and be quietly using your bandwidth for their own purposes. Usually the trojans and backdoors bypass the usual stack monitoring so their traffic doesnt show up in the usual tests...
