How does one address Sarah Ferguson?

Since her divorce (and after she lost the style Her Royal Highness due to new letters patent being issued) her name is simply "Sarah, Duchess of York" She is neither a princess, nor a duchess, rather simply a wife a a divorced British duke. I suppose Mrs. Duchess of York would therefore be appropriate, as she has no other title. Your Royal Highness would be incorrect, as she does not have this style any longer. Your Grace (the style for the wife of a duke) is not longer appropriate, as she is no longer the wife of a duke. And, she was never a princess in her own right, but by marriage.

I suppose one could call her Duchess out of courtesy, but it is kind of like calling someone by their last name. She is not The Duchess of York any longer.