Alison
DJB has answered your query about Essex Regiment archives. The Palmer Casualty roll is a very large publication which encompasses the South African Field Force Casualty list and the Natal Field Force Casualty list. It has all the imperfections of the originals and it is a fact that some casualties are not recorded. Some don't like the Palmer roll; I do because it is at hand and very often is useful in resolving a query such as yours. For example, there is no "H.Young" listed as a casualty for the Essex Regiment. If you wish, I can send a copy of the relevant section of the Palmer roll.
In the fog of war, it is possible that a man could serve with another unit on attachment and examples I have seen, where the man became a casualty, the attachment was noted on the medal roll. Again, not infallible. It is also possible that a man could have his forename recorded wrongly on the casualty roll. As a matter of interest, I have an example of a man in the Essex Regiment where his QSA featured a wrong Regimental number and fore-initial! I did check the Essex Regiment South Africa medal roll for any evidence of that in your case. There was no evidence to support a mistake in forename.
So, unless you can tap some family memory about forenames, or find any documentation, letters, etc., it is difficult to take your query much further.
Regards
IL.