Just pleased I could help a little.
The National Archives sent me a direct link to download piece number WO 100/128, the QSA Medal Roll for the 19th to 22nd battalions I.Y. that is actually 2 pdf files. John appears on page 84 of 208 of the first pdf file (page 76 of the actual roll), the page entitled XXth Battalion Imperial Yeomanry Regiment and dated 10 April 1901. His entry reads:
Regimental Number: 15730; Rank: Private; Name Robertson J.A.S.;
..entitled to clasps: Cape Colony, Orange Free State; Remarks 78th Coy I.Y. South Africa.
All you need now is a copy of his enlistment & discharge papers and hope that in addition to his place of birth, his enlistment also gives his address at the time of enlistment and his discharge gives the place in SA where he was discharged.
I don't know if Ancestry have images of the British Army Campaign Medal Rolls from WO 100, but if not I can send you copies of pages from the Queen's & King's SA Medal Rolls that I've quoted from. If you want them contact me at
jeffp DOT h AT bt[No spam]internet DOT com
Change obvious text and remove all spaces and [No spam]