John Mortimer jr sold a house in Hilton Road in 1927 to St Anne's College, and there is extensive reference to the Mortimer House on Hilton Road. I have not found any reference to the Mortimers in Hilton, but as a property agent he is likely to have had many properties at various times. He donated part of erf 13 Burger Street to the city to create Leighton Street, for example, so he had a town house as well as the property on Hilton Road.
As far as I can piece together John Mortimer snr was born in Bristol in 1850 (he is listed in the 1881 census). He was a wine & spirit merchant with the firm Mortimer & Clune. His line is listed back to a John Mortimer born in Credition, Devon in 1794. He converted to Methodism, signed the pledge and sailed off to South Africa in 1890, because he had been to the Transvaal in 1871 on doctor's recommendation to relieve his asthma. In 1893 John Mortimer snr started the conveyancing firm Bale and Mortimer with William Bale in Pietermaritzburg.
John Mortimer jnr would have been 15/16 when he arrived in South Africa, and I have no record of any schooling - he probably joined his father's firm. He is listed as a clerk by profession in his marriage certificate. The younger brother, Clifford was 5 when he arrived in South Africa and went to Merchiston Prep school, followed by Michaelhouse, Natal's version of an English public school, then Cambridge and a distinguished legal career. His son John Mortimer was the playwright and barrister born in 1923.
My grandmother often spoke about "uncle Cliff" who was clearly the shining light of the family. Sir John Mortimer, the playwright, met her when he visited South Africa in 1993 to reconcile with his South African family at the end of Apartheid. JM was a notable anti-Apartheid campaigner.
The Mortimer family is said to have disapproved of John jnr's marriage to Mabel Jackson. They were married in 1897. I have no idea why!
This is pieced together from passages in JM's biography, what records I have managed to find, and bits of family lore. It's a best guess at this stage!