Hi Kate
Starting from the top (furtherest back in time):
I have found a John Sandilands (b. circa 1705) who married Jean Proven (b. circa 1708/09) and they had at least 3 children who were Ann b.1736, Marion b.1738 and William b 1741. My William Sandilands who married Jean Pittloe is most likely this son of John and Jean as I have found no other likely candidates for his parents even after years of exhaustive searching.
William and Jean had the following children: Agnes 1766, Jean 1769, John 1771 and Magdalin 1774.
John (b.1771) married Ann Tofts (b.1769 and daughter of John Tofts and Jean Campbell). The Meikle name comes from John Tofts paternal grandmather. This surname has many spellings and so causes confusion.
The John Sandilands (ploughman) you ask about, is actually your gggggrandmother's brother, not her grandfather. Her grandfather is the John whom I am sure married Jean Provin in Dirleton, East Lothian, in May 1735.
Hope this helps. Email me on colleen1@xtra.co.nz if you would like me to fill you in on other parts of this family.

Kind regards
Colleen