I think I have cracked the Robert Harry Jackson mystery.....
QMS Robert Harry JACKSON appears to be 30168 Trooper Robert Harry PAYNE, 24th (Westmorland & Cumberland) Company, 9th Bn. Imperial Yeomanry. He sat his law examinations under the name of Jackson, but later practised as a solicitor under his birth name, Payne. The Incorporated Law Society Report states that Jackson served for only 9 months and the dates (March to November 1901) tally with those given in Payne's service papers. Various records show that both "Jackson" and "Payne" were born in Ashton-under-Lyne in 1859/60. However there is only one actual birth registration, and that is under the name of Payne. His mother died in childbirth or shortly thereafter, and at the age of one Robert is living as a Jackson with his "widower" father, Henry Jackson. However, I can find no record of a marriage between his mother and father, and I suspect he was born out of wedlock, being given his mother's name at birth. He reclaimed this name sometime after 1889, having taken his law exams and married under the name of Jackson.
His father, also a solicitor, was, on 25 April 1885, imprisoned for 5 years for the theft of £1,000, with which he had been entrusted by a client. It appears that by reverting to the name of Payne, Robert hoped to distance himself and the business from his disgraced father. Law examination records show that Robert was working for his father’s firm when he gained his degree in 1889. His father had been released from Chatham Prison four months earlier, but presumably was no longer allowed to practise as a solicitor.
It seems likely that the Incorporated Law Society continued to use the name Jackson because this is the name that appears on his law degree.