[eluser]oddvalue[/eluser]
There arn't any htaccess files on the server, I'll ask the admin what the uri rewriting config is but as I say it works for WordPress, b2evolution and Drupal... don't know if it makes a difference that CI uses MVC...
I managed to get at the log file, seeing a whole lot of this:
[17/Oct/2009:16:46:34 +0100] 90.193.210.108 - - "GET /index.php/blog/ HTTP/1.1" 404 - "-" "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/532.0 (KHTML, like Gecko) Chrome/3.0.195.27 Safari/532.0"