When Leslie/Elsie was baptised her brother William was baptised at the same time. His date of birth 8th January 1882. He appears in 1939 war register with wife and son. The same dob and a wife Gwendoline (Jones).
This couple married 6th July 1902 Llandeilo Talybont, his father recorded as William Thomas--occupation Carpenter, believe father was also a witness.
William & Gwendoline also appear on 1911 census.
THOMAS, THOMAS HENRY mmn JONES
GRO Reference: 1916 D Quarter in PONTYPRIDD Volume 11A Page 1222
1921 census look for
THOMAS, ELIZABETH ANNE mmn JONES (Lizzie Ann in 1921)
GRO Reference: 1906 M Quarter in CARDIGAN Volume 11B Page 3
THOMAS, WILLIAM HUBERT JONES
GRO Reference: 1915 D Quarter in PONTARDAWE Volume 11A Page 1828
ADDED
William Thomas jnr both in 1911 & 1921 records his pob as Tregegar, this comes under the Bedwelty registration district. He was actually born as William Thomas Williams 8th January 1882, mother Rachel Williams --domestic servant, info taken from official gro birth record.
Rachel married William Thomas sq 1882 Bedwelty. This marriage certificate should name their respective father's and occupations.