I work for Wordcandy.net, and we recently contacted the English press agency who handles the rights to Heyer's stuff. According to the man I spoke with, the only thing stopping a Heyer adaptation is the fact that most producers believe there wouldn't be a market for one.

Behold:
http://wordcandybooks.blogspot.com/2009/03/georgette-heyers-film-adaptations.html

Clearly, the producers are insane, but there you have it...