Hi Brian,
Yes, that's the well-known tune I used to hear a lot on the folk scene in the 60s. I can't say I remember Martin singing it.

I will have a closer look for the source of the descending tune after the weekend. I have lots of recordings I can listen to of traditional singers. I'm a bit busy at the moment preparing for Saturday's Broadside Day in Manchester. Meanwhile it must be on one of Ian (Jock) Manual's 2 albums.

Ian,
Questioniing the validity of any theory is something dear to my own heart. However, as I said earlier a ballad written perhaps in the 18th century based on half remembered facts from a century earlier is a common occurrence. Until a better match comes forward for the song's origins it is enough to say the ballad could be based on the 1634 story even if some of the facts don't stand up to detailed scrutiny. I doubt very much if Johnny Armstrong, Earl of Cassilis Lady or any of the border ballads match up with the facts accurately. They have all been romanticised and politicised.