Thank you so much for your help ohrenkino.
actions > actions > new > (enter name) > new > Format Value
Field = _FILENAME
Format String = the second half of what I had before
...then...
new > Format Value
field = _DIRECTORY
Format String = the first half of what I had before
...gives me a single click action that does exactly what I was doing before, but moves all the non-audio files too. As you said, folders can be joined without a problem (any duplicate filenames in the two folders cause a normal windows file overwrite warning to be displayed). You can't split folders this way, but I don't need to do this at the moment. My previous method would happily split folders, so I guess I can add that as a third step if ever needed.
Many thanks again.
David.
P.S. no idea where that link in my first post came from. It should have been...
/t/14613/1
...which is a current thread discussing almost the same thing, but I did not understand how it was relevant until you explained that it would work for what I was trying to do.