I should think you will have to get the birth certificate for one of John and Mary's children to get Mary's maiden name, and Elias looks to be the best bet. I think this should be his birth registration, from FreeBMD:
Births Mar 1883 (>99%)
Jones Elias Pontardawe 11a 639
(You could specify parents' names when ordering if you want to avoid getting a wrong cert)
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