To add to the above:
If I change the code in the Mobi file from "<br>" to "<div> </div>" then that comes through "unchanged" to the ePub, and displays correctly as a blank line in ADE, but unfortunately it then
doesn't display as a blank in Mobi

.
Would it be possible for the Mobi to ePub conversion code in Calibre to change "<br>" to ""<div> </div>" to avoid this problem?