Thank you very much for posting this, and I will definitely follow up the South Africa connection, which might solve the mystery for me.
I have a feeling that your notes might mix together two different, but related families.
Based on what you have told me, I have found a Margaret HEPPENSTALL baptised at Ecclesfield on 12 June 1820 to Thomas and Mary Heppenstall. She seems to have married Thomas ROBINSON in the Halifax Registration District in the December Quarter of 1850, and they had four children: James Edward (1851); Margaret Ann (1853); William Henry (1859) and Alice E (ca 1863). There may also have been some others.
Margaret HEPPENSTALL had a brother named Philip HEPPENSTALL, baptised at Ecclesfield on 23 Feb 1823, and he married someone named Mary. Together they seem to have had two children, both born in Liverpool: William, born about 1854 and Mary born about 1856. William, as you said, married Mary Elizabeth ROBERTS in the March Quarter of 1879, and his sister Mary married Henry Albert ROGERS; they had the two sons: Leonard and Laurence. I can't immediately see any connection with the name Graham, either as a forename or as a family name, but your suggestion that the ROGERS family went to South Africa is definitely worth following up.
If you can offer any more details about these families (HEPPENSTALL; ROBINSON or ROGERS) I would be really pleased to hear from you.
Thanks again for the helpful lead.
Kind regards
Philip Candy