For my Windows 8 application I require some data files (probably xml) which contain the content of the app, these are the same for each user and should only be updated with as new versions are released.
What is the appropriate mechanism/location to store/read these files?
If you're bundling the data with the application, and it doesn't vary for user or version, then simply include the files as content within the application itself. You can then access those files from the local package via the FileIO class. Take a look at File access and permissions in Windows store apps and the ms-appx:// protocol.
Alternatively, you could store the data in the cloud and access it via an HTTP request from your application. It's another moving part, but it would enable you to update content without necessarily putting out a new version of the application.