This is a very late response to your message,which I seewas posted in 2001.In fact I read it some time ago, and it is only now, after a visit to my family home in South Africa, that things suddenly fall into place.
I found a letter from my grandfather, James Cockburn Thoms to my father Alan Thoms (who died in 1987) in which he gave details of all his brothers and sisters. The letter was written in 1946. They are unmistakably the same family as your grandfather's.
In the letter he lists:
Robert - died at sea, 1873 (This we always knew - he was swept off his father's ship off Cape Horn, not a whaler, a tea clipper. Our family has a model of the ship.)
John - died Glasgow Charlie -died Dundee Sarah - died Glasgow, had a son in Canada David - died in Wales Willie - ran away to sea as a boy - settled in Seattle, died approx. 1931 James - my grandfather died in South Africa in 1947 Isabella - still living in Scotland when the letter was written.
My father's elder brothers were Hamish and Malcolm Thoms and it is probably they who had corresponded with Elizabeth Thoms - but they certainly were not farmers!
I live in Denmark, but return to South Africa frequently. Let me know if I can be of further help to you and your grandfather,