He appears on the birth index as Frederick Magnus Janes, b. June quarter 1873, Pancras district, London.
Could this be them in 1881?
Name Relation Marital Status Gender Age Birthplace Occupation Disability
Emily HARTRIG Head M Female 28 Marylebone, Middlesex, England Dress Maker
Frederick M. HARTRIG Son U Male 8 St Pancras, Middlesex, England Scholar
Catherine HARTRIG (Daur) U Female 4 St Pancras, Middlesex, England
Dwelling 8 Bennett Street
Census Place London, Middlesex, England
Family History Library Film 1341040
Public Records Office Reference RG11
Piece / Folio 0184 / 87
Page Number 30
A 'Magnus Hartvig' is listed in the Post Office Directory of Middlesex, 1874, 4 Suffolk Villas, Pellet Grove, Wood Green.
Kelly's London Suburban Directory, 1896. [Part 4. Southern: Localities] has Mrs Eliza Hartvig, ladies outfitter, 41 High Street West Norwood.