Most likely you know about this website, but on the off chance you do not, this may serve as a resource:
http://www.stjohnhistoricalsociety.org/Also, you may want to look at the book, Night of the Silent Drums by John Anderson. While it is fiction, it was well researched and details the plantations, life and the slave rebellion on St. John in 1733. It is a little before the time period of your ancestor, but it gives a good idea of the conditions and life on St. John.
Hope this helps.