Thank you both gentlemen. My Zoetwater is likely to be the one near Calvinia in the Ca[e Colony.
David - you clinched it with the newspaper extract - how D.C. Trollip, F.I.D. became B.Trollop, as per the newspaper, is anyone's guess. I am taaking full advantage of the Find My Past free weekend which is where I dug up that Denham Cameron Trollip, QSA (CC/OFS) Eastern Province Horse and KSA (Field Intelligence Dept.) was WIA there,
The inimitable El Ne has also conjured up a map of the area (below)