Sotham was gone from the Battalion by this stage, I was just about to suggest it was Capt Sharples (adjutant), who was also killed at the redoubt.
It looks as though it is the same fella who is next to Elstob in the pic on pg 201 of Stedman's Pals.
From the Batt History:
'Captain Sharples, the Adjutant of the Battalion, was prominent for his cheery engagment of the men, both by word and deed, always on the firestep where the enemy pressed ever-closely, firing with a quiet confidence, until he was killed.'