Hi Maureene & Jonathan,
I already know the names of the officers who were serving with Hodson's Horse, both before and after 1864 however, the problem arises in particular with the officer who was Commandant in 1864, owing two the fact that C. H. Mecham was in command from 1863 to 1864 (after the death of J. P. Caulfield in 1863), and he was then replaced as Commandant by H. L. Campbell, also in 1864.
Being of fairly typical composition the photo shows three British officers, and the officers seated in front are obviously of some importance and in my view would be the Commandant, and the second in command, with the third officer very likely the adjutant; as there was only four officers at that date, the only person who would be missing would be the medical officer.
So for 1864 when the photo was taken, without doubt, the officer wearing the helmet is the commandant however, he could be one of two people, Mecham? or Campbell? The other seated officer is the 2nd in command who was Dayrell.
So I was hoping that someone with a knowledge of the personalities involved, might be able to identify the face of the person wearing the helmet, and thus the answer in part to my original question.