What package have you bought? Have you checked the website for server status, since some areas are experiencing "difficulties"? I have the 600k and it is fine (except the mail has been down for the last few days).
Are you running heavyweight programs in the background ?Something like Kazaa will slow everytnig down to a standstill at times.I have NTL 'broadband' which is fine most of the time(Kazaa excluded) but will slow down noticeably around 6pm and Saturday mornings when I presume people on dial-up come on-line ( only guessing)
I don't know why, but sometimes if I log out and reset the set top box it all works better when I go back on line. I know when it needs doing not just by the internet speed but also because some of the TV stations start refusing to work.
Mine was slow too also had the tv problems but rang NTL and they were having problems - lost the internet all over the weekend and the TV but it came back on mon night - no problems so far though. Give NTL a ring it might be your box.
It's worth trying a speed test - this site has a good one. http://www.dslreports.com/stest/0.
Plus
it gives ideas on how to speed up your connection. Look at the 'tweaks' section.