For your basic question everything seemed to be clear. I was trying to explain about his "B' classification if you are talking to me.

Jan 1916- Battalion went to Mesopotamia.
Cecil might have went with them. He might have gotten wounded or some other injury that sent him back to England.
April 1916- England

May 1916- 8th Service Camp Btn. Attached to Trench Mortar. He might have been in a capacity of supplying the Trench Mortar; but not in a capacity that didn't fall out of his "B" status.

If he could not serve on the lines. He might have been in some back-up capacity of supplying or Labor work for the Batteries behind the lines; that is if he could not be on the lines.

I think you need his Service Record or if he survived the war and was awarded a pension. Either one of them would give you a lot of insight about what happened. I don't remember if you had said if you had searched for them. They would be on Ancestry as you probably well know.

God Bless and God Speed,
Danny