Rosie - Samuel Hay, was the son of James & Helen (Eaton) Hay, so he would have been brother of John Hay. As to the marriage records, I have not found anything there. I was fairly sure the marriage listed in Kirkintilloch was not him, as I know that the family lived there in 1901 census, however, they had relocated to Govan and were living on Drive Rd in 1905. At that point, John would have been 16 and possibly still living with his parents. As you say, he is a difficult relative to trace.