Michael - thanks so much for your response. It got me looking a bit deeper and yes I think I missed a generation. I think I see now that Archibald married Isabel and had 9 children - one of whom was Duncan (born 1809 died 1880. Duncan married Mary (Oig) MacDonnell daughter of Donald MacDonnell of the 5th concession Lochiel in (1883). They had several children - among them was Angus Alex who married Sarah McDougall. I am positive that they are my great grandparents. They had 9 children (Hughie Archie (d1975); Cassie (m Bob Williamson); Mary Belle (m Eugene Corcoran); Sarah Anne (m Fred Jenks); Duncan Alex (d un-married); Stanley Alex (d un-married): Angela (m a Dr. Lachapelle); Edith (m John Hoke?); and Agnes (my grandmother) who married Alex Chisholm of Lochiel (my grandfather).
So it looks like I just need to insert that one step - i.e. Duncan (son of Archibald and Isabel) who married Mary McDonnell and had, among others, Angus A McMillan who married Sarah McDougald. (these would be my great grand-parents
Does this now make more sense?