Frank Purslow, in The Wanton Seed (E.F.D.S., 1968) had this to say about Sovay:

A much printed song on broadsides where the heroine's name is usually Sophie or Sylvie; in later copies there is an additional first verse which adds absolutely nothing to the story.  The song is currently rather popular at the folk-song clubs and I would like to point out that the tune is not -as I recently heard an earnest young singer explain- in 7/4 time: the three notes at the beginning of 2nd and 4th complete bars are triplets, i.e. three notes in the time of two.  Apart from one notable exception, Riding Down To Portsmouth, the rhythm of 7-in-a-bar does not exist in English folk song.  It is just possible that the original timing of the tune was in 9/8, although I don't think so.  I would say that the song is a typical product of the 18th century pleasure gardens.

The version given in The Wanton Seed (from the Hammond & Gardiner collection, noted in Dorset in 1904) has substantially the same tune -in 3/2- as the one usually heard on modern recordings, though the triplets Purslow refers to have now disappeared; this is probably due to A.L. Lloyd, from whom almost everybody currently singing the song got their version at one remove or more: "In a couple of places I've added a pinch of spice to the rhythm", he said.  

Malcolm