Though this is a slightly slower response than Monica's, I thought I'd add some information about Mosside, for your interest.
Mosside was part of a larger holding called Gricetail (Grystale, etc; Grystoull in the 1590 Act Anent the Debateable Land), located on the south side of the present-day village of Canonbie, which was divided in late 18th century into Priorlinn (also Priorslynn etc.), Mosside and Hollinhurst during what was called the Age of Improvement. The shift in farming from the collective to individual tenancy resulted in many landless families and much movement of the farm labour force.
My gg grandfather was born in Mosside. His father, John (b.1756 in Gricetail), was a tenant farmer there when he got married in 1791. John died at Mosside in 1835. He was the last of the family to live on the steading of Gricetail, where it was said that the Armstrongs had lived for 317 years.
Wyllies are listed at Priorlinn (William, 41, ag. labourer) in the 1841 Census. There were also Wylies in adjacent Closses and Forgebraehead, and Byreburnfoot, not too far away. But no William Wylie, in any of those places, matches in age to the father of your Ellen, as listed in the 1851 census. Perhaps he was away at that time, though.
Cheers from someone whose ancestors ploughed the same ground!
Mosside
