There is a book written about this:

John Jack, the slave, and Daniel Bliss, the Tory: ... - Google Books Result

I came across it while researching Daniel Bliss the man who wrote it for John Jack.

http://books.google.com/books?id=8qc_AAAAYAAJ&pg=PA9&lpg=PA9&dq=epitaph+of+John+Jack&source=bl&ots=Ae13Qrp2dE&sig=VQi32v6w4lrakpG8SyH08EpU1Os&hl=en&ei=2VvLS-j8Job49AT3vOW9BA&sa=X&oi=book_result&ct=result&resnum=6&ved=0CBUQ6AEwBQ#v=onepage&q=epitaph%20of%20John%20Jack&f=false