I wonder if the informant for Helen's birth actually knew the biological father's occupation. If James
ROBERTSON was living in the Parish of Montrose then these are all I can find in the 1841 census.
1841 Census
ROBERTSON James 35, school teacher boarding in Union Place, Montrose
ROBERTSON James 35, farmer, living with what is obvioulsy his wife and family Charlton, Montrose
ROBERTSON James 21, ship's carpenter living in Queens Lane, Montrose
ROBERTSON James 15, caster app living in Bridge St Montrose
ROBERTSON James 15, carpenter's apprentice, living in Ferry Street, Montrose
Of course he may not have lived there at the time of the census.
Unfortunately family lore about a newspaper advertisement which we in Australia have no hope of researching, doesn't really help locate your person of interest

Are there archives for the Montrose newspapers of the era?
Cheers

Cando