You're right, Jon.
Caught me on the hop there. I'd forgotten it was written on.

The only obvious clue as to when it was written on would be 'to YE tune of ....'. Presuming it was genuine, I don't think they were still using 'ye' in writing in the 18th century but I may be wrong on that.

Susan,
The motif was used on a broadside ballad in 1685.

I have some original correspondence on 'William and Margaret' between Chappell and Ebsworth. I'll check it out when I have time to see if it throws any light on the matter, also Chappell's and Ebsworth's notes in Roxburghe.