Hi Craig, welcome to SilverStripe :)
Does www.websitename.com/ work?
This may have something to do with mod-rewrite, I think 2.3 doesn't need it anymore, but you get funny urls if you dont use it. Is it definately enabled? Also check your .htaccess file to see that it has all the correct SS stuff in there...
Also just to be sure, you are installing SS at the root domain, not www.websitename.com/client/ right?