Tentative identification
There is no Sidney Theobald in the RFA (Royal Field Artillery) named in the Commonwealth War Graves Commission database or the Soldiers Died database. There is a medal roll record for a Sidney Theobald in the Royal Field Artillery (Service no. 35703) but the entry gives no clues about birthplace or residence or date of death.
However, in the 1911 there is a Sidney Jack Theobald, born around 1896, living at 21 Milford Street, Clapham. He was a telegraph messenger boy. This person matches with an entry in the Soldiers Died and CWGC databases for Sidney Jack Theobald, who was born in Clapham and lived in Wandsworth.
Service no. G/40069
Private, Queen's (Royal West Surrey Regiment), 2nd Battalion
Died on 26 October 1917, aged 22
CWGC: "Son of James and Charlotte G. Theobald, of 532, Wandsworth Rd., Clapham, London."
Remembered at Tyne Cot Memorial, Zonnebeke, West-Vlaanderen, Belgium
The engraver of the panels may have made a mistake (this was not uncommon), or there may be another Sidney Theobald who was indeed in the RFA but for unknown reasons was not included in the standard databases.