Thanks to Chris McK's post here for this:
You have to use the 64 bit CMD prompt as administrator.
1. Create a shortcut to C:\Windows\SysWOW64\cmd.exe on the desktop
2. Right click the shortcut and Runas ADMINISTRATOR
3. Enter full path and file name: e.g. C:\Users\[YOUR NAME]\Downloads\SQLManagementStudio_x86_ENU.exe