Some things in life should be as simple but aren't! After years of installing various versions of Visual Studio you would think I would be prepared for battle but Visual Studio 2012 still managed to get me!
tl;dr Use this iso file to install Visual Studio 2012 Pro, the web installer doesn't work!
(As a side note, I long for the day where installing something on Windows is as easy as it is on *nix. I can install a whole operating system and associated programs in the time it takes to install Office! Alas, that is a separate post!)
I then decided I would try a clean install using the old fashioned iso provided by Microsoft. I tried to uninstall Visual Studio 2012 and .... it failed! (I can see a patten forming here.) I tried various things and finally after three tries managed to uninstall it. By try various things I mean I rebooted, I tried to kill various processes that looked like the installer was orphaning and clearing out %temp%.
I then installed this iso file (via the excellent daemon-tools). Forty minutes and another coffee later and it worked and installed.
You would think that in this day and age the web installer would be fail safe. It should know what packages you need download them and install them in a fast and successful manner. Come on Microsoft, it's not hard!