Right there is a happy conclusion at the end of this thread. In the end I reflashed both the box downstairs and the problem box upstairs and lo and behold I can now FTP into both boxes again. I don't know what caused the problems but it might be a lesson to folk who are running more than one box on the same network - do not restore from an old back up if you are planning on upgrading to the newest versions of VIX!
Many thanks to all those who replied to this thread and who tried to help me out
Many thanks to all those who replied to this thread and who tried to help me out