XML File not found using Drupal directory in field

Another weird error - I can access my xbel file using the fully qualified url in the "Path to XBELF bookmark file" field, but if I try and use any of the directories below I get a file not found error ("Bookmark file not found! Check path to your xml file!").

I was under the impression that the default behaviour for drupal system directory fields was to start from the (fictitious) /drupal-root/ directory and work down the hierarchy, hence this is possibly part of the confusion. In other fields in core, (e.g. www.example.com/admin/settings/file-system) directories are listed starting in that manner (on that pages, the default "File system path" is "files" with no trailing/leading slashes.

Hopefully I'm not missing anything obvious here!