Ok EDIT.
I managed to find a solution which is bizarrely hidden and obscure.
http://osxdaily.com/2010/12/28/rotat...n-orientation/
You need to hold COMMAND+OPTION down
when clicking on the icon in the System Control Panels to open the displays control panel. Only then will rotation controls be enabled and visible for a secondary monitor!
This must be a new "feature" in the more recent OSX 10.6 release(s).