Just for further info........
In the early sixties, there was a Low Level route from Cape Wrath/Garvie Island to Stroma in the Pentland Firth.
This obviously passed just to the north of Dounreay and on occasion we were warned of a DH Heron 2 in it's vicinity.
I found out later it was operated by English Electric, transporting staff from Leicester.
Maybe another brick in your wall ?