Steve.
   Correct me if I am wrong.Did Prof.Child not give John, 6th Earl of
   Cassillis the honour of being his chosen villian in the ballad?
   If he did then he was surely in error & his scholarly collabs.
   sorely to blame.
   Paterson in Vol 2 of his History of Ayrshire Pub.1852 prints a
   letter from this Earl to the Earl of Eglinton dated 15th Dec.
   1642 appraising the latter of the death of his dearly beloved
   bedfellow & inviting him to attend the funeral on 5th Jan.1643
   The Earl & his Lady were married in 1621.
   The Earl married his second wife in 1644 & a son of this
   marriage became 7th Earl in 1668.         
   Sorry to cause you so much trouble checking books which may be
   the wrong source. I have been telling the story for a long time
   now without having to consult the source.
   Joe.