If you have download speeds in the 60's, I would check a few things first, before phoning ntl.
If you have an Ambit modem (ntl:home 100/120), you can check which service your modem is configured for, by going here:-
http://192.168.100.1
(password & username = root)
Clicking on 'Operation Configuration' will show you which service you are on, under 'Maximum downstream Data Rate'.
Then I would check that windows is set up properly for your BB speed by going here:-
http://www.dslreports.com/tweaks
and running Tweak Tester II to see if your RWIN is set correctly. If not, download DrTCP from the same site & set RWIN to a value it recommends.