Philip José Farmer reveals further information about Singleton in Tarzan Alive. Singleton was born John Clayton, the son of Harold Cecil Caldwell-Grebson, who had changed his name to Harold Cecil Clayton on marrying Chrysogon, daughter of the 3rd Earl Staveley and becoming heir in right of his wife. Harold Cecil was himself the grandson of Sir Oliver Tressilian, the pirate at the centre of Rafael Sabatini's novel The Sea Hawk.
According to Farmer, Captain Bob returned to England after various adventures accompanied by his Waziri wife (this contradicts Defoe's account), and their son John Clayton (his father having confirmed his identity) became 4th Earl Staveley and (in right of his wife, Elizabeth Gracia Caldwell-Grebson), 13th Baron Grebson of Grebson. In turn they were the parents of John Cecil Clayton, 5th Earl and 14th Baron, and 1st Duke and Viscount of Greystoke.