Thanks for your interest avm. I've not done any work on this family for some time, I'll have to go back to my tree and look up the info that I have? I know that my grandmother's mother's maiden name was Walker, perhaps this Mary could have been her sister, and after she died, could have taken in the child William Dreghorn, hence him being in the census. Very interesting! My grandmother didn't have a sister named Mary, but her mother Elizabeth Walker, my GGmother, who married Robert Douglas, may well have.
Now you've got me going, I need a new focus, and this Dreghorn one has puzzled me for a good few years! That'll keep me quiet on a cold winter's day!
Thank You! I'll let you know how it goes!