This site has always helped us in the past with our problems. We know from first hand experience that they are good.
http://www.dfes.gov.uk/bullying/
This site by the Department for Education and Skills is really useful, containing practical information about what you can do if you're being bullied, with advice for teachers, friends and family, too!
Childline is talked about so much that people tend to take it for granted. Unless you have used their service, you cannot imagine how comforting and helpful they can really be. Check out their website!