In Burkes Landed Gentry I spotted an entry for Isabel because of her marriage to Ian Greig. It listed her father as Col. Campbell.
Had one of my John Campbell’s been in the army prior to becoming a rancher in Argentina? One might assume from the title Col., provided in the well respected Burkes, that this was the case but it was not to be.
I searched the army lists for an age but without ever discovering a Colonel Campbell that would fit with other information. I moved on to other leads.