In a theatre of war, Siege Batteries of the Royal Garrison Artillery were usually under command of a Heavy Artillery Group (sometimes called an RGA Brigade), which in turn was usually under command of a Corps headquarters. On occasion, a battery might be under direct command of the Corps headquarters or even an Army headquarters. The allocations shown in the images below provide a good guide but I must point out that I have often found errors and omissions in the dates given. For an accurate picture including the locations and activities of the batteries, consult the war diary of the Heavy Artillery Group or Commander Heavy Artillery of the Corps concerned.
Source
Allocations of Siege Batteries RGA. National Archives piece WO95/5494. Crown Copyright. Photographs are my own digital copyright, marked to help identify any unauthorised misuse of the images.
Links
Allocations of Siege Batteries of the Royal Garrison Artillery to higher commands (index page)