Ferdinand Charles Meurant [Convict "Minerva" 1800] is John's father-in-law.
Here is a link to the son mentioned William Edward Meurant
According to https://www.gettyimages.ae/detail/news-photo/emperor-joseph-ii-holy... this I think this picture https://media.geni.com/p13/67/cc/37/5f/5344483ab5df0d79/ferdinand_2... is actually Emperor Joseph II, Holy Roman emperor and king of Bohemia and Hungary. Painting by Jean-Etienne Liotard,