There is this possible marriage:
Jonathan Jones and Mary J Griffithe
1st Qrt 1918 - Merthyr Tydfil
Vol 11a and pg. 1056
And possible birth:
Iris Jones
2nd Qrt 1918 - Merthyr Tydfil
Mother's Maiden Name: Griffiths
Vol.11a and pg 1503
You need some actual certs to verify further, Daniel. Names are common and easy to make mistakes really.
The Merthyr Tydfil marriage is the only one with groom Jonathan, but there are other Jonathan Jones marrying elsewhere at that time.
Monica