An image of his enlistment form for the Grenadier Guards is available on findmypast (behind paywall) and gives his birthplace as Saint Mellons, Cardiff and age as 18 years 11 months on a date of 25 May 1912. If you change the profile to Open I'll upload a "fragment" of this image to get around the copyright restrictions.
His age implies he was born around June 1893 and sure enough his birth record is found in Q3 1893.
"England and Wales Birth Registration Index, 1837-2008," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:2F93-VKW : 1 October 2014), Dumane Swinfield Thomas, 1893; from "England & Wales Births, 1837-2006," database, findmypast (http://www.findmypast.com : 2012); citing Birth Registration, Cardiff, Glamorganshire, Wales, citing General Register Office, Southport, England
The GRO index records his mother's maiden name as "Dumane" although he clearly wrote his name as "Du Mayne" on the enlistment form. Finding him in the 1901 census was a challenge but he's "Dumdine S Thomas" aged 6 (7 actually) the son of John and Mary J Thomas.
"England and Wales Census, 1901," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:X9TB-R65 : 21 May 2019), Dumdine S Thomas in household of John Thomas, Risca, Monmouthshire, Wales, United Kingdom; from "1901 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing St Woollos subdistrict, PRO RG 13, The National Archives, Kew, Surrey
This enables us to confirm his parents marriage: "England and Wales Marriage Registration Index, 1837-2005," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:2DGM-6VK : 13 December 2014), Mary Jane Dumain, 1873; from "England & Wales Marriages, 1837-2005," database, findmypast (http://www.findmypast.com : 2012); citing 1873, quarter 4, vol. 11A, p. 404, Newport (Monmouthshire), Monmouthshire, England, General Register Office, Southport, England
It's wrongly categorised as being in England because of historical confusion surrounding Monmouthsire but it was in Newport, Wales.
I also see you've been slightly "bigamous" yourself in creating two profiles for Du Mayne https://www.wikitree.com/wiki/Thomas-29868 and https://www.wikitree.com/wiki/Hobbs-6235 so please just merge them together.
After his prison sentence Du Mayne seems to have wasted little time in fathering more children with Rosanna Growel/Growell/Grovell (her father was Dutch so it was probably spelled with a W but pronounced V). I haven't found a marriage record but one son's birth was registered as Ivor Swinfield Thomas (mother Growell) in 1927 but by 1939 is living with a elderly couple (not his grandparents) and is now Ivor S Grovell suggesting the marriage (if there was one) has broken down.