Thanks for the follow up - I will give it some thought.
I have a John Pressnell (born 1842) who married Harriet Swan (1843-1893).His brother William Wells Pressnell married Harriet's sister Eliza Swan (1846-1876).Does this fit at all?
See also John Wm. Presnell born Sevenoaks 1848 (also in the IGI), John Wm. Pressnell born Canterbury 1876, John Wm. Presnaile born Maidstone 1888.However, I think that your John Wm. must be pre civil registration.