Problem partly solved. I put the same query on Tradsong Forum and Steve Roud replied personally. No wonder I couldn't find it. The title is 'The Inconstant Sailor and Distracted Maiden' or at least the first 2 verses are. The only known copy so far is in the Madden Collection with 14 stanzas. Although it's a Bedlam ballad the last 2 verses don't fit and are either from another ballad or both pieces derive from an earlier ballad, possibly one of those Dicey Marshall pieces of the mid 18thc.