Update: With the leads you have provided me, I have found the following:
An ebook available on the web, The Great Boer War by Arthur Conan Doyle, in Chapter 32 "The Second Invasion of Cape Colony", the following: "The Western Boers were commanded by Judge Hertzog of the Free State...and about twelve hundred men....Meeting with a stout resistance...they abandoned the attack and turning away from Coleberg, they headed west, cutting the railway line twenty miles to the north of De Aar. On the 22nd they occupied Britstown...and on the same day they captured a small body of yeomanry who had been following them. These prisoners were released some days later." This incident seems to be the same one referred to in the email posted by ianelder. I am happy to have both references. It is somewhat a strange coincidence that my Great Uncle's brother later married a woman in the U.S. with the maiden name Hartsog, sometimes spelled Hertzog!
Also, I now have in my possession of a book (poor condition) titled The War in South Africa, A Narrative of the Anglo-Boer War From The Beginning of Hostilities To the Conclusion of Peace, by Captain A. T. Mahan, 1902. I found no specific reference to the Imperial Yeomanry unit or the incident regarding my Great Uncle, but it does provide some general information and a nice map of the war zone, which was helpful to me.
I also have sent an email to staff at the Maitland Cemetery in hopes that they may be able to provide me some info about my Great Uncle's burial and grave. I haven't received a response yet except an acknowledgement.
Sorry for the length of this posting, and thanks again for the help you have provided.
Dan