Thanks for that Matthew. I went to the library earlier today and, although I didn't see the collection itself, the index states merely that it contains "several hundred (Harkness) broadsides".
I'll bite the bullet and reserve a PC for half a day sometime soon. I'll be happy to look up Barbara Allen for you.
I'm also interested in finding out more about the man himself. Although born in Carlisle in 1817, Harkness spent most of his life here in Preston, but there's almost no biographical info about him at all. He deserves a bloody statue - as it is no-one is even sure when he died (sometime between 1885 and 1890).