Through the Royal Welsh Fusiliers website I found some more information about two brothers Benjamin d. 1955 and Cadwaladr d. 1978, both buried in the Llandinorwig churchyard with parents Annie and Cadwaladr.
William was born 05/1893, d. 24 Nov 1918.
There is a sister no one can locate due to the married name change. My next step was to start hunting family trees.
Also have this bit:
Roberts, William Cadwaladr, Pte 40158
Born Dinorwic c.05/1893. Quarryman. Enlisted 22 RWF 02/02/1916. Landed 28/07/1916. Posted 1 RWF 30/07/1916. L/Cpl 04/11/1917. Cpl 08/05/1918. Shrapnel wounds legs 08/05/1918, adm 21 CCS, 9 S/Hosp. Re-joined Bn 26/06/1918. Adm PUO 22 F/Amb, 9 CCS 16/11/1918. Influenza. Died 24/11/1918, age 26. Buried Giavera British Cemetery.
Next of Kin given as Mr Cadwaladr and Mrs Annie Roberts, Bron Fferam (?), Dinorwic, Cwmyglo, Caernarfon
Further sources of information: local council administration, local historical society, contemporary local papers and generally knocking on doors.
[Just in case: F/Amb = Field Ambulance, CCS = Casualty Clearing Station, S/Hosp = Stationary Hospital, PUO - Pyrexia of Unknown Origin]