I just came across a 
nice article that appeared in 
Mystery Scene Magazine last summer. Not too much new information, except that it shares a little about Mary Stewart's reaction to getting an honorary degree from her alma mater, Durham University:
In a letter to me that same year, Lady Mary Stewart said of the 
experience, “It was indeed wonderful to be remembered—and in such a 
way—by my old University.”
And apparently the author of the article asked her if she would write another book:
“No new book! I know better than to try at (nearly) 93!” she wrote me in 2009.