Oh no, I've just found that census, Agness born 1826, it was abigail that was born then!!!! So Agness is John's sister in law, can't believe I got it wrong. Must stop researching when I'm sleepy 😴.
So, maybe Colin was correct and she did marry gilchrist, maybe they're not from durisdeer, but I think they were as per names of the parents..
.. Will quickly look for a Walter.....