With regard to my search for a William Hendrie/Hendry/Henry who fathered two children with Mary Mathieson, William (abt 1850) and Margaret (1853), it now looks suspiciously like William Hendry (b Riccarton, 1809; d 1879, Kilmarnock).
He was married to Mary Ann Smith from Ireland (m 1834). They had no children and Mary died after the 1851 Census but before 1855.
William then married Mary Drysdale (again from Ireland) in 1858. They had four children, Mary Hendry (1863), William Hendry (1867), Jeanie Hendry (1870) and John Hendry (1876).
I believe on the basis of location and occupation, but cannot prove, that William Hendry had a relationship with Mary Mathieson between his two marriages.
If anyone has any knowledge of this William Hendry I would be delighted to hear from them.
There is information on the internet regarding the parentage of William Bruce Hendrie (b 1850) which give the father as William Wyllie Hendrie. I believe this to be wrong.