Hi Phil I found this which might be the marriage of WH Miller in 1919 which would make his birth date 1872 and Jim would have been born in 1868. Sounds about right. Click on the link
https://familysearch.org/pal:/MM9.3.1/TH-267-11596-169994-95?cc=2063749&wc=M9WM-PR1:n1689800448 and here is some more info.
William Henry Miller b 1872 was married firstly to Catherine Causier in England and then absconded to join the Army and eventually settling in South Africa. Not sure if he got divorced from his first wife? He died in 1947 aged 75. As can be seen above in the link he then married Wilhelmina Hansina Mathilda Anderson born 1891.
DEPOT NAB SOURCE MSCE TYPE LEER VOLUME_NO 00
SYSTEM 01 REFERENCE 2443/1947 PART 1
DESCRIPTION MILLER, WILLIAM HENRY. S/SP MILLER, WILHELMINA HANSINA MATHILDA. STARTING 19470000
ENDING 19480000.
And here is your grandfather and his wife's estate notices.
DEPOT NAB SOURCE MSCE TYPE LEER SYSTEM 01
REFERENCE 101/1969 PART 1 DESCRIPTION MILLER, WILLIAM HENRY. BORN IN LADYSMITH, NATAL. S SP MILLER,
MILDRED BEATRICE SUSAN. DECEASED ESTATE. STARTING 19690000
ENDING 19690000.
DEPOT NAB SOURCE MSCE TYPE LEER SYSTEM 01
REFERENCE 1353/1969 PART 1 DESCRIPTION MILLER, MILDRED BEATRICE SUSAN. (BORN CRICKMORE). BORN IN DURBAN,
NATAL. PRSP MILLER, WILLIAM HENRY. ESTATE NO. 101/1969. DECEASED ESTATE. STARTING 19690000 ENDING 19700000.
It is a shame I never found this info before as I was in Natal and could have gone to the archives to look up these notices.
I found the memorial plaque for WH Miller and his wife Mildred located at Stellawood cemetery in Durban.
Regards Derek