Looking around for information and came accross an excellent post from Rob Clifton detailing the establishments of Heavy and Siege Batteries, as well as the HQ elements.

Source: HOW MANY MEN IN A SEIGE BATTERY

August 1916 Heavy Artillery Battery and Ammunition Column (6 x 60-pounder guns)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 8 Serjeants, Farrier-Serjeant, 3 Shoeing-smiths (incl one cpl), 2 Saddlers, 1 Wheeler, 1 Staff-Sjt Fitter, 1 Smith, 7 Corporals, 8 Bombardiers, 110 Gunners, 71 Drivers, 7 Batmen.

Attached: Serjeant AVC, 3 Drivers ASC.

Ammunition Column: Subaltern, 1 Serjeant, 2 Shoeing-smiths, 1 Saddler, 1 Wheeler, 1 Fitter, 1 Corporal, 2 Bombardiers, 12 Gunners, 35 Gunners as Drivers, 1 Batman. Attached: 1 Driver ASC.

September 1916 Siege Artillery Battery (4 x 6" howitzers, horse drawn)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 7 Serjeants, Farrier-Serjeant, 3 Shoeing-smiths (incl one cpl), 2 Saddlers, 1 Wheeler, 2 Smiths, 2 Trumpeters, 6 Corporals, 8 Bombardiers, 90 Gunners, 47 Drivers, 7 Batmen.

Attached: Armament Artificer AOC, Serjeant AVC, 2 Drivers ASC.

Transport personnel: Subaltern, 2 Serjeants, 2 Corporals, 21 Drivers, 1 Batman.

September 1916 Siege Artillery Battery (4 x 6" howitzers, tractor drawn)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 5 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 6 Corporals, 6 Bombardiers, 100 Gunners, 6 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 3 Serjeants, 3 Corporals, 38 Drivers, 2 Batmen.

September 1916 Siege Artillery Battery (4 x 6" guns)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 6 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 6 Corporals, 8 Bombardiers, 120 Gunners, 6 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 3 Serjeants, 6 Corporals, 71 Drivers, 2 Batmen.

September 1916 Siege Artillery Battery (4 x 8" howitzers)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 5 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 6 Corporals, 8 Bombardiers, 120 Gunners, 6 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 3 Serjeants, 6 Corporals, 64 Drivers, 2 Batmen.

September 1916 Siege Artillery Battery (4 x 9.2" howitzers)

Battery: Major, Captain, 4 Subalterns, BSM, BQMS, 5 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 6 Corporals, 8 Bombardiers, 120 Gunners, 6 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 3 Serjeants, 8 Corporals, 83 Drivers, 2 Batmen.

December 1916 Siege Artillery Battery (6 x 6" howitzers, horse drawn)

Battery: Major, Captain, 5 Subalterns, BSM, BQMS, 8 Serjeants, Farrier-Serjeant, 3 Shoeing-smiths (incl one cpl), 3 Saddlers, 2 Wheeler, 3 Smiths, 2 Trumpeters, 6 Corporals, 8 Bombardiers, 135 Gunners, 70 Drivers, 8 Batmen.

Attached: Armament Artificer AOC, Serjeant AVC, 3 Drivers ASC.

Transport personnel: Subaltern, 2 Serjeants, 3 Corporals, 29 Drivers, 1 Batman.

December 1916 Siege Artillery Battery (6 x 6" howitzers, tractor drawn)

Battery: Major, Captain, 5 Subalterns, BSM, BQMS, 7 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 8 Corporals, 8 Bombardiers, 150 Gunners, 7 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 4 Serjeants, 5 Corporals, 52 Drivers, 2 Batmen.

December 1916 Siege Artillery Battery (6 x 8" howitzers)

Battery: Major, Captain, 6 Subalterns, BSM, BQMS, 7 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 8 Corporals, 12 Bombardiers, 180 Gunners, 8 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 4 Serjeants, 10 Corporals, 90 Drivers, 2 Batmen.

December 1916 Siege Artillery Battery (6 x 9.2" howitzers)

Battery: Major, Captain, 6 Subalterns, BSM, BQMS, 7 Serjeants, 2 Smiths, 1 Wheeler, 2 Trumpeters, 8 Corporals, 12 Bombardiers, 180 Gunners, 8 Batmen.

Attached: Armament Artificer AOC.

Transport personnel: 2 Subalterns, 3 Serjeants, 13 Corporals, 121 Drivers, 2 Batmen.

Were they self-sufficient? The answer is generally yes, at least until late 1917 when their allocations to HAGs/Brigades became more permanent. For such matters as medical and veterinary cover they would be placed under the care of appropriate officers by the Cdr HAG or Cdr Corps HA of the Corps to which they were attached at the time:

August 1916 Headquarters of Corps Heavy Artillery

Commander, Brigade Major, Staff Captain, Vet Officer, OC ASC (Major), Captain ASC (workshops).

4 clerks, 1 staff-serjeant, 12 artificers, 30 rank and file.

August 1916 Headquarters of a Heavy Artillery Group

Lt-Col commanding, Adjutant, Orderly Officer, Medical Officer.

Serjeant-Major, 1 Serjeant, 2 Clerks (one a sjt), 1 Corporal, 2 Bombardiers,

13 Gunners, 2 Drivers, 2 MO's Orderlies, 3 Ptes RAMC, 2 M/cyclists ASC, 5 Drivers ASC MT, 1 Interpreter, 4 Batmen.