Always authored by Guest

I can't seem to get the Authored By field to override the default 'Guest' user.

I presume this is something to do with users/ roles/ permissions. I have set up a "FeedBot" user who has a "News Bot" role with all permissions relevant to the feedparser module (in addition to authenticated user permissions). All my feeds have the Authored By field set to 'FeedBot' but the items appear to be authored by guest...

What am I missing?!