Hrob123.... This is a very late response.... 14 years perhaps? But, this may or may not be of any assistance. I too am having some difficulty tracing possible ancestors who may have emigrated to South Africa. Which is part of the reason for this very late post.
In a nutshell, John Johnston(e) was born in Annan, Dumfries-shire, Scotland in 1840, (According to the 1841 Scottish Census). He moved to England and the Isle of Man, where he married Jane Edgar in June 1859. Eventually they had 5 children and moved to an address in Walker Street, Birkenhead, Cheshire, England. The 1881 Census records him as a 'Beer House Keeper', (a pub landlord).
After 1881, there is no verifiable information on John, his wife Jane or any of his children, save my G.G.Great Grandfather Douglas. By 1891, John's brother Peter and his wife Mary and their family have moved into the premises, and there is no signs of John or any members of his family.
Over time I have tried to trace John and his family, but there is no concrete information that tells of what happened to them. However, a female named Martha Johnston did apparently marry an Owen William Rolfe, who, it would appear travelled to South Africa. Although I can find no emigration information on any of them.
So, if any of your distant relatives had the surnames Johnston, Edgar or Rolfe, there may be a slim chance that they are originally from Annan in Dumfries.
I know it's a long shot, but hey, the information on these things does come from the strangest connections.... Good luck.
P.S. If anyone can shed any light on the likely places to look for the family, it would be much appreciated.