John Evans being a somewhat common name there are rather a lot of marriages between a John Evans and a Mary in the region at the relevant time. However, I think the marriage of John Evans to Mary Gwyne in June 1818 is the most likely for the following reasons:

• The marriage took place about 3 weeks before the birth of John's first child.
• One of the witnesses at the marriage was a William France whose wife Jemima was a witness at the marriage of a Charles Frost to Mary Ann Wright in 1828. I have yet to find a relationship between Charles and "our" Frost family. He could be a brother to Edward (whose father was Thomas) but I have not found a baptism for Charles unless he is the Henry Charles Frost baptised in 1804 with father John. If this is the right Charles then he is not a brother to Edward but could be a cousin. Henry Charles was baptised at St Giles in the Field which is not that close to Islington where, according to the 1851 census, he was born.