Hello Ken,
Markham's medal actually appears to have been re issued to him, or, I suspect, the Cape Government, on the 15th of November 1907, after one of the clasps had been added from the supplementary roll of Nesbitt's Horse, he did serve in the Midland Mounted Rifles as 31067 Trooper and is clearly shown in both WO100/259 as well as WO100/262.
Whilst Nesbitt's Horse was raised for "general" service within the SAMIF, it did spend most of it's time in the Cape Colony, the Midland Mounted Rifles was a Cape Volunteer regiment, both were kept rather busy, in particular, after the second invasion and of course, they had quite a number of Cape rebels to deal with too.
All surviving rank and file papers for both are at Kew, those for the MMR are in WO126/91-92 and for Nesbitt's Horse, WO126/95-98.
You clearly have issues with this particular medal, the clasps are very common, you will have no problem getting original examples, however, the real issue here will be the claw, pin and suspender, you could use that of a BWM, as indeed, the mint did with very late issues, but, it would be far better to simply use the correct original.
Anyway, bon chance Ken,
Regards Frank Kelley