To guest Dave, it would indeed have been strange, not to say treasonous, if Nell Gwynn had sung a song in praise of Blake.
"To their eternal infamy, the Stuarts disturbed Blake's grave. Blake had opposed the King's (Charles I) trial. He had disapproved the usurpation (by Cromwell.) When he found the sword prevail against law and right, he abandoned politics... giving up his genius to the service of his country against its foreign enemies. - Seventeen months after his landing at Dover, a command was issued by (Charles II) to tear open the vault, drag out the embalmed body, and cast it out into a pit in the Abbey yard."

Of course, there is nothing yet in this thread to suggest that the song was actually written earlier than the novel in which it appears, i.e. two hundred years after the Civil War.