I just clicked on the Sigma link only it didn't take me to the Sigma site. I then tried the
http://www.sigma-imaging.co.uk version and it redirected me to Tamron's site!
Sigma UK is
http://www.sigma-imaging-uk.com/
Sigma USA is
http://www.sigmaphoto.com/
Sigma Japan is
http://www.sigma-photo.co.jp/english/
[Edited to correct my mistake in the URL]