Hi MS
As I said, the Francis Grierson is conjecture but as near as I can find.
If you remember that on Thomas's marriage certificate, his father's names is entered as Francis Dalziel - who doesn't exist. I surmised at the time that Thomas's father might be a Francis and he had said 'Francis' to the registrar, who wrote in 'Francis Dalziel' as that was Thomas's surname. He also called one of his sons Francis. The same registrar entered Mary as Dalziel, M.S, Stewart, so there was obviously a confusion. Thomas may have been trying to hide his illegitimacy or the clerk got the information muddled.
It was common for illegitimate children to be 'known as' their biological father's name - Jane would no doubt have known. Later, he changed to his mother's surname, which was his 'legal' surname as Mary was unmarried at the time.
When we find a possible Thomas with his grandmother and listed as Grierson, I went to look for a Francis Grierson and found this one - the only one in the area and of an age (b.c. 1829).
We can't say for certain that this Francis Grierson is Thomas's father but we can hazzard a good guess.
If it was my tree, I would pencil Francis Grierson in as a strong possible but only that.
Regards
Gadget