Our PDF build is dependent on XEP, which is installed on my desktop, so up to now I've needed to be here to build it (since it was a scenario run from inside Oxygen). I've now converted this into an ant task which can be run remotely at the command line. Lesson learned at some cost of time: you can't use this:

  <arg value="-fo ${foFile}"/>

Instead, you have to use this:

  <arg line="-fo ${foFile}"/>

Otherwise XEP doesn't find the FO file, and assumes the fo is coming from stdin; it then complains that the root element is not fo:root.