You could have a play around with the Broadband Buddy Settings on your online NOW account and under custom type in the url address for the website in the allow permission box.
I would also be tempted to try a reboot on your NOW Router to see if that makes any improvement.
If they are legit websites and not blocked by the ISP then maybe try phoning the NOW Broadband Team for further assistance and help.
Have you got their telephone number?
Is it happening on all your internet browser devices or just something like on your PC.
If on a PC i don't know if there is anything on this link internet article helps in anyway.
Anyway here's the telephone number for the NOW Broadband Team on the link page below (they close at 8pm and reopen at 8am).