I downloaded the stable version from http://www.silverstripe.org/assets/downloads/SilverStripe-v2.3.1.tar.gz
This file is 6.6mb and does not include a core/Core.php nor does it include install.php
Only a "sapphire" directory and going to the url does nothing, zip, nada, what am I missing ?
What else is included and can someone point me to a clear insctruction page ?

Are you sure you are extracting the file correctly ?

It does contain an "install.php" file and a lot more..