Sorry for the delay in replying Ruth - I've been contacting one or two family members in search of additional relevant information but without much success I'm afraid.
The story seems to be as follows:-
The mother and father - David Wilson (b.1859d.1942) and Elizabeth Agnes Wilson (nee Larmour b.1860d.1940) appear to have left Belfast and lived in South Africa from about 1928-1931 and then returned to Belfast. They had 7 children (not sure of exact order) - Mary Larmour (my Grandmother b.1883d.1976), David, William, John, James, Elizabeth Agnes (b.1900d.1960), and Herbert (d.c.1966).
David (son) appears to have been around 19 when he went to SA (Johannesburg early 1930's - a guess - during the depression) and eventually, with others, founded the Pheonix Foundary. John, I understand, settled somewhere on the east coast of SA. William also went to SA and settled in Port Elizabeth. James also seems to have settled in Johannesburg. Elizabeth Agnes and her husband John Richard White also spent some years in SA before returning to live in N. Ireland. Mary Larmour and Herbert remained in N. Ireland.
Not sure if any of this will help search.
Thank you for your interest