Hi Ian,
Sorry, I wasn't thinking entirely clearly there.
Before you downgrade, you can try resetting the password for the account that phpMyAdmin uses with this:
UPDATE mysql.user SET Password=OLD_PASSWORD('phpmyadmin-pass') WHERE User='phpMyAdmin-user';
substituting the appropriate values for the user and password. (You'll need to be logged in as root to do this.)
So long as you don't reset that password using PASSWORD() or the GRANT command, you should be able to use it with phpMyAdmin.
I'm a bit to the south of your old stomping grounds (I'm in Logan/Woodridge -- y'know, in the high-rent district ;) ), but I pass by the Yeronga train station from time to time and sometimes change trains there when I'm headed to or from Indooroopilly or Toowong or Kenmore.
Jon Stephens
MySQL Documentation Team @ Oracle
MySQL Dev Zone
MySQL Server Documentation
Oracle