Hello Rory,
Well, I think it is a good medal, moreover, if I did not, I'd bloody well say so, I suppose, despite Natal being huge and very sparse as far as the populace went and indeed, still goes, the issue of the clasp entitlement is clear as far as I am concerned.
Calverley's Natal clasp could only have been earned up until the 11th of June 1900 and is, very clearly, rare, to his regiment, despite him actually serving in January 1900, clearly, he was not actually in the Melmoth Field Force, had he been, he would have received the Relief of Ladysmith clasp, Eshowe is well east and slightly north of Escourt, but, both Melmoth and Ulundi are far further to the north, Natal was and remains huge, members of this regiment had a tough job to go and do, the enemy would certainly not have thought twice about a few cattle raids, they hated the Zulu people, perhaps, with good reason, although, the ones I have met, I actually rather liked.
There was also the possibility of the enemy actually crossing Zululand from the north and managing to get as far south as to being able to raid Pietermaritzburg, you had to defend the Colony and the Natal Royal Riflemen were there when they were needed, it must have been a little daunting for some of it's members to have to remove or cover those magnificent white helmets, but, they did play an important role in the defence of their own Colony.
Interestingly, from memory, the Magistracy in Natal had made it very clear that they expected any such cattle raids to be swiftly "sorted out" by the native police, perhaps then, unsurprising they recruited, paid and more importantly, armed a small portion of the several hundred new constables when the war broke out, they certainly could not have counted upon the Volunteers to cover all that ground, as already mentioned, it was a huge area.
Generally speaking both a Natal clasp and a Relief of Ladysmith clasp, each on their own, to any Natal Volunteer is pretty scarce, they simply don't turn up in any quantity.
I was both a little interested and rather surprised by your comment regarding this particular medal's past and it having done the rounds, all I can think, is simply, that they were unaware of just what the regiment was actually doing in the Anglo Boer War, because, had they been, I am sure it would not have been on the market.
Never mind, silly buggers, their loss and your gain!

Kind regards Frank