I have found John Masters in the 1851 Census living at 98 Monnow Street, Monmouth, with house keeper Ann Walley. He was unmarried, born in 1808 Stanton Prior, and a wine merchant.
I am certain that these two married in 1858 at Merthyr Tidfyl.
Now that I am going to welcome him into the family I thought I would try to find him in 1841 and then in 1861.
There are quite a few John Masters born c 1808 on 1841 census who are married, I'm looking but can't find one with an occupation of a wine merchant
This is the marriage entry you are referring too
Jones Mary Merthyr. T. 11a 554
Martin William Merthyr Tydfil 11a 554
Masters John Merthyr T., 11a 554
Walley Ann Merthyr T 11a 554
It would help if he could be found on 1861 census confirming who he married
I see from the 1851 census
https://www.familysearch.org/ark:/61903/1:1:SGTP-LP3Ann WALLEY was born c 1814 Chelsea
and her Baptism shows father's name as Thomas, they can be found on 1841 census living in Newport Monmouthshire her father's occupation an Accountant, her sister Ellen was bp same place
on 19th Dec 1813
Ann Walley Baptism 28 Jun 1812 St Luke, Chelsea, Kensington dau of Thomas Walley & Elizabeth
Edit ~
I don't know whether there's any connection here, three John Masters together
1841c
https://www.familysearch.org/ark:/61903/1:1:M7MW-X9QJohn age 70 occ Merchant
John age 40 occ Ind.
John age 15