I have managed to get further in my attempts to make myth archive work. It was erroring out at the mythtranscode stage and I found the solution in this thread

http://ubuntuforums.org/showthread.php?t=610856


basically, I had to remove the .ICEauthority file from my user's home directory, then mythtranscode would run.

I'd love to know why? It really worries me how a command line program like mythtranscode can fail due to a file, which according to what I've googled, is something to do with KDE.