Thanks for your efforts, Henry.

I'm beginning to think there may be no more than the original verse. The lack of attribution in Finden's Ports and Harbours suggests that it might be traditional, but it could equally well be the product of one of the book's compilers. The complete lack of result from Google line-by-line searches might suggest that the verse never appeared in any other printed source - or that such a source hasn't yet been scanned and made available to the internet.

Ross