I assume that the simple reason that Heyer hasn't been filmed is that she is off the radar of a lot of the types of people who commission dramas, and that even when she comes on to it, she is dismissed as "Oh, only middle-aged women would watch that." Remember, TV exectutives were completely taken aback at the success of the first series of "Lark Rise to Candleford" - they didn't see that there was a large audience for a well-written, well-acted, well-produced, relatively gentle but not soporiric Sunday evening drama...