Hello Richard,
Well in that case I would suggest that he was indeed the same man, not a common name, if he joined in 1894 and was really rather good, he could have attained the rank of Lance Sergt by 1900, actually being killed on the 22nd of February 1900 at Wynne's Hill, Natal, would have been a sad end too, but, that fate awaited so many in South Africa.
WO96 comfirms that Hezekiah Monks joined the Militia, the 3rd Battalion, Kings Liverpool Regiment in August 1894 and then joining the Army, the South Lancashshire Regiment, in October that same year.
Interestingly WO100/189 page 131 says "Deceased" but looking at the published casualty roll, I would have to say killed in action, when the 11th Infantry Brigade, 5th Division, assaulted the enemies positions on that particular line of hills, the hill in question is named Wynne after the commander of the brigade, Major General Wynne
The obvious thing to do now is look at him and his family in the RG series, once you have identified him as being the only possible with that exact name, in that particular place and in that particular time frame, then you can move forward, he was killed rather than discharged, so his papers will not be in WO97.
The thing you really should do now, is to look at what you know, you have a name, you have a image of this man, now as long as you are sure it is indeed your great uncle, have a close look at his uniform, depending on the particular order of dress in the photo, the Prince of Wales Volunteers were rather differant than the Kings for example.
Also try and link him and his family with his death in South Africa, the obvious way to go would be to look at the press of the day, not the national newspapers, although, you would find him, but, the regional and in particular the local one, I would hope that a careful search would give you an article in which his parents and address will all feature together with his death in Natal.
Anyway, I wish you bon chance,
Regards Frank