Hi Pete,
it seems the plugin can not find your .gpx file, see here:
troubleshooting
You say there is a .gpx file in your images folder.
Try to either rename it to track.gpx, or change the
invocation code to match your actual filename (i
hope it's not just ".gpx", this will likely not work!)
If the track still doesn't show, then there may be an
issue with the access rights to the files in that folder.
You can set "Show warnings" in the "Advanced options"
to "Yes" and see if there's any warning or error messages
when you open the page with the map.
hth & cheers, Frank