Since posting my "story" on Rigg on another Forum I've had input from Hiram, a Scot, who postulated as follows apropos the kilt Rigg is wearing.
Does anyone have another view on this or a source where one could find out more?
The photograph of Lt Rigg in the kilt is interesting, they are all wearing kilts with the khaki apron covers, also diamond diced white and red 'Hose Tops' (foot less socks) with khaki highland pattern spats, the khaki tunics are of the Scottish cut away pattern, the kilt set look like that of the Gordon Highlanders?, for a signal section they appear to be seasoned troops and extremely well armed with the 1895 Lee-Enfield Magazine Rifle Mk1, and enough ammunition to start a war.
From the photograph I would say he was posted to this unit as the signals officer. Some more research on the photograph may through up an answer, I would post it on the Anglo Boer War site and Victorian War Form they are very helpful.
Regards