Peter and Neville - thank you both very much for your input - together with my "map finder" Elne Watson, we have come up with the attached which seems to be a good fit for the action. I think you are both spot on - the skirmish must have taken place at Winterkraal near Smithfield and thus near to the CC/OFS border.
Perhaps those who identified the spot as being in the Cape Colony were guilty of a misapprehension. I am quite happy to claim the Smithfield Winterkraal as the place Dow earned his DCM.
Regards
Rory