Hi again Simon,
I'm trying to establish:
Where and when the couple were likely to have married - looking at the 1861 & 1871 Census on Freecen Mary's age is a good bit younger than William's . She is 41 & 52 respectively - making her year of birth circa 1819/1820. Therefore they are most likely to have married late 1830s or sometime into the 1840s. The year of birth of their first child could help narrow that down.
Also names of the children (including middle names)and the order of birth can give clues.
Many Scots followed a naming tradition - which you may or may not be aware of

so apologies if I'm telling you something you already know

1st son - named after father's father
2nd son- named after mother's father
3rd son- named after father (unless the name was already in use above)
1st daughter named after mother's mother
2nd daughter named after mother's father
3rd daughter after mother ( unless already in use )
Middle names were frequently the mother's maiden name - other family surnames (granny's maiden names for instance).
There are quite a few William Stephen/Steven/Stephens in Scotland around the years we are looking at so any extra information could help narrow them down.
Do you have the deaths of William and Mary? You have them up till 1871 Census

Were they both dead prior to the 1881?
Sorry for the inquisition but with the info from your first post William and Mary will be very hard to find

Looby

Added - should have said , of course, that not all Scots followed the naming trad. strictly.