Have you tried changing your dns servers? It worked for me. Most people are using googles
instead of the automatic plus.net ones. You can change them on your computer, or if you log into your router and look at your adsl settings there is probably an option to manually add them instead of automatic. Just put in the google ones.
If you can't face it you can try the free dns jumper software
which will most do it for you with a few button press's and also find you the fastest ones available if you wish.
Anyway using someone else s dns servers fixed it for me today. The plusnet ones were failing half the time in the afternoon and all the time in the evening.