Jack, you are making the common mistake of giving the lady what she asked for, rather than what she wants.

When Sue asked for something 'not a song' she meant she wanted something slow and sad and intellectual that would contrast with another piece, 'a song', which probably has verses and a refrain and which you might hear around the camp fire.

How could anybody say that a certain slow air has never been sung? It might be 200-300 years old, and nobody knows how far it's travelled or who wrote it.

As for the Great Silkie, I seem to recall that the Judy Collins Songbook says the words were set to an old air. If anybody claims copyright to that, it's not legit. (If my memory is right.)

Sue, do you read music? You'll probably find that your combo will work better if the two pieces are in the same or related keys, and if they have similar meter. For example, I find with my friends that we can seque from 3/4 to 3/4, and 3/4 to 6/8, but 3/4 to 4/4 is uncomfortable.