Thanks Artie76, perhaps this is another piece of the puzzle.
After a pause of several years - searching for Irish family - I have taken up my Arnot(t) family once again and searching thru the Ayrshire archives came upon your reply.
This David Arnot - McGilchrist connection at this 1841 christening sounds very promising. I believe Falkirk is not far from the Campsie area where my family lived. The McGilchrist father John might be Agnes's brother and David might be John Arnots brother although he's not listed along with John and siblings from the family list I got from the LDS site familysearch.
Worth investigating further. :-)
Chris in NZ