Sign in web page appears for anonymous uses access Doc Library on SP 2013 public website

Hi,

Within a Office 360 SharePoint 2013 Preview public facing web site I have created a document library and added some excel documents to it. Everything works ok when I try to access the document library and open a file with my credentials, but when an anonymous user access the same document library a Sign in web page appears. Anonymous users have access to the website but not the document library. When I do a search as an anonymous user the documents within the folder appear and I can open the files, but when I try to access the same files by first opening the document library the Sign In web page appears. The document library permissions show that Anonymous users have View Items permission (inherited permissions).

How do I setup Office 360 SharePoint 2013 Preview so anonymous users can access any file within a document library without a sign in page appearing.

Many thanks
CEStar