I've researching these also, and have come across a few things. One, was it Isabel or Isabella? I keep coming across McGilcrest, and it came to my attention that the spelling may have been changed from Menteith. I was always told they we're a part of the Stewart clan, and I just went from there. Also, I may have stumbled across parents, trying to narrow it down. I found a christening on 10 Jun 1743 in clackmannon, parent Walter Monteith and Katherine Robertson. I also found a William and Margaret Campbell. Like I said, trying to confirm.