Graham, it is possible to edit style parts individually without affecting all the others if you start off in Style Creator.
So with the style loaded to the KB, start Style Creator and select the part (eg. D) you want to edit. You may have to press Exit first in Creator to access the selection screen on Ty5, I can't quite remember.
Then start Mixing Console and edit as needed eg put volume to zero to "mute".
When done push Exit to get back to Creator and save the style, or select a new part for editing and repeat the above.
John