I think I recognise that file size.
You can either ftp to the Site Files area of the site, this is the folder "1" in your moodledata folder.
The other numbered folders correspond to the other courses on the site. You can identify which is which by hovering your cursor over a link to a course on the site page e.g. http://mysite.co.uk/moodle/course/view.php?id=19. The files for this course reside in folder 19.
If the backup or restore links in the administration block have been used to access a files area there will be a "backupdata" folder in the files area, this might be a useful place to ftp your back up file.
If you ftp to the site files area then only Admins will be able to restore the course.