Finally I think I have got it and have committed changes to TimeTextCtrl so that NTSC drop-frames works correctly for both dragging and inc/dec of the frames, secs etc..  If anybody out there who has NTSC drop-frames experience I would appreciate a test - for example one minute down from 00:10:00;01 (or is that 00;10;00;01?) is conventionally 00:08:59;29 or 00:09:00;02?
Will try and remember to update the comments in th file shortly.