There looks like a peering problem with some NL routes at the moment on NTL, as Chrysalis says other ISP's don't seem to be suffering as we NTL users are so it looks like NTL's fault.
Mornings are fine, from about lunchtime onwards it all slows down until the evenings are awful, a good place to see this in action is the Giganews speed connection tester page:
http://www.giganews.com/test_connect.html
Try it at different times of the day, hopefully NTL will fix this, but I'd guess they won't hurry themselves as they never seem to spot peering problems until its too late.