James, thanks to your recent feedback on another thread, I've been able to fill in more gaps. I'm delighted.
With - as it appears - Harriet McMillan being my 4th great-grandmother, I guess this makes us 5th cousins as well. Harriet's daughter Mary (Sarah) was the wife of Alexander Ban Kennedy.
Now I need to search for the parents of Jane McDonald, the wife of Alexander's son Duncan.