Thanks for all your answers. The 21st Lancers seems like an excellent suggestion, I have actually got Churchill's account of Omdurman in his 'War Correspondence 1895 -1900' So yes, it would seem like a good idea to link my character roughly to Churchill's experiences both in Sudan and the Cape.
I could perhaps do away with the the idea of his being a groom and instead just have him join the Lancers at the earliest opportunity. Would you happen to know the minimum age that a man could be enlisted into the cavalry in the 1890's?
I could perhaps have him ignore his obligation to the reserve in order to get him to the Cape.
The case you cite of an OR 'offering violence to an officer' is very useful indeed. In my fiction I imagine that my character as an OR with some 'personal history' with a man who is the same age as him but a Captain.
The situation in which I want to place my character is one where the SALH are asked to take a hill, knowing that they have re-enforcements coming some way behind, The Boers are at the top of the hill, but their artillery is further down and shelling the troop (who have dismounted) from an angle. I imagine a situation in which the CO is a Major who is picked off by a sniper, and the second in command is a Captain who bears my character a grudge, and whom non of the men respect, because of his lack of judgment etc. So when he is called upon to take command, the Captain gives orders for an assault just when the Boer artillery have found their range. Several men are immediately killed by shellfire including the Captain's best friend. He goes to pieces slightly, and my character ( a sergeant) takes it upon himself to get the officer to take proper command, and then when this doesn't work, he takes it on himself, and leads a successful assault on the hill just as the artillery re-enforcements start get a bead on the Boer artillery. The outcome being that the officer decides to save his face by accusing my character of insubordination.
I'm sure you'll all be having a good laugh at the unlikeliness of this situation but hell, it's fiction!
Tell me what you really think of this - is it in any way believable?
Thanks for your help!