Bit More,
Perhaps this may help on his surname and on finding any siblings, note his mother's surname suggests twice married.
Name: JAMES, ARTHUR MOSSOP
Initials: A M
Nationality: Australian
Rank: Private
Regiment/Service: Australian Infantry, A.I.F.
Unit Text: 30th Bn.
Age: 28
Date of Death: 20/07/1916
Service No: 689
Additional information: Son of Albert James and of Sarah Susan Ashton (formerly James), of 240, Guinness Buildings, Draycott Avenue, Chelsea, London, England.
Still searching
JM