Anyhow Joseph Stephens married Hester Wyatt 1708 in Wiltshire and had a son Joseph in 1710. This son Joseph Stephens died in 1718 and it was noted that he was the son of Hester Slade.
Joseph the father died in 1720 and the Slades had their four children baptised one month later. Obviously the Slades were living in a de facto relationship having never married but having four children in the previous ten years.
Looking at the parish registers for Hilperton and nearby parishes, I can't see a marriage for Mervin SLADE and Hester - have you found one at all (before or after the death of Joseph STEPHENS)?
I also note that the parish register for the marriage of Joseph STEPHENS on 6 Jan 1718/19 looks like he marries Hester
WATTS of Seend.
Interesting that Joseph STEPHENS was buried in Trowbridge "a married man". On the same day a William STEPHENS was buried "a young man" - a relation?
Why did they wait? Maybe they didn't want to advertise their unmarried relationship, maybe it was some sign of respect to the original husband? Just wondering?
It does seem likely that the 4 children John, Thomas, Ann and Sarah were baptised after Joseph STEPHENS died, perhaps the family wanted to ensure they were baptised with family name SLADE? I don't think they were concerned about their defacto relationship being known - son Joseph's burial shows that the priest knew Hester SLADE had previously been married to Joseph STEPHENS.
