Thanks to your ggrandfather I had a fine education

The NAAIRS (SA Archives website) is "free". However it is only assists in establishing if there are indeed records for i.e. Thomas Muir, and provides the reference numbers. You would have to go to SA (Cape Town in this case) to view the records. The SA Archives are an amazing resource, and one of the best organized in the world. You could also try searching for a professional genealogist in SA (try the SA Genealogical Society) and pay a fee? The Aussie currency is strong against the ZARand. At this stage there is no genealogical look-up service through the SA archives. As you have probably already found, Thomas Muir bought a great deal of property, over 100 entries are registrations of mortgages, and apart from reference to a dinner party organized for him, there doesn't appear to be much more of interest that would assist you with his birth date in Cape Town. A search of all public records in SA (the various data bases are listed) might yield more (i.e. his appointment as Superintendant of Education). I didn't look for his children. Good luck.