I tend to agree with Steve about the storyline of this ballad. The incest story is often simply an extension of the more basic one.

I looked up the Oxford songs, as suggested by Alan (thanks for steering me towards two more great ballads to add to my 'to learn' list; and thanks to the Digitrad for supplying tunes!) I can see the similar themes.

I guess it's the reference to the father in the 'Willow Garden' that sets this ballad apart from the others. I don't recall coming across that element before.