Thanks again for taking the trouble to reply, Ken. I don't have Ancestry (as yet) but I do have details of Donald Calderhead, my grandfather's brother. The CGWC certificate records:
In Memory of
Gunner DONALD CALDERHEAD
40304, 108th Heavy Bty., Royal Garrison Artillery
who died age 27
on 16 February 1917
Son of John Calderhead and Mary Wotherspoon Calderhead, of Wishaw, Lanarkshire.
Remembered with honour
ETAPLES MILITARY CEMETERY
I have tried to find out what a Gunner's responsibilities are, without success. I must be looking in the wrong places.
I'm keen to learn anything about this man as my grandfather was disowned by his family when he married my grandmother (nothing sordid, just plain old sectarianism) and so I have a whole branch of family that I know nothing about. Any information you can offer would be gratefully received. I do appreciate the time, patience and effort you have spent helping such a sadly-lacking novice.