Hope this isn't a red herring:
1841 Census
Name: Dougald McFarlane
Age: 20
Estimated Birth Year: abt 1821
Gender: Male
Where born: Scotland
Civil Parish: Govan
County: Lanarkshire
Address: Thomson Lane
Occupation: Saw Sharper
Parish Number: 646
Household Members: Name Age
Margt Chrystie 20
John Howie 35
David Leighton 30
Dougald McFarlane 20
Isabella McFarlane 4
Margt McGaw 2 Mo
If you allow for the rounding off on ages for the adults, this could be your Dugald with Margt Chrystie for his wife. Isabella McFarlane's birth would have been in 1837 which agrees with the 1851 Census. Also this Dugald is a Saw sharper!
Carol