I know you have asked for an 1891 census look up however you state you know nothing more on him around 1873+. It seems that John Manley was the step son of Henry Allens this information is within the 1881 UK census (Devon region)
Dwelling: Sheppards Valley
Census Place: Dunkeswell, Devon, England
Source: FHL Film 1341514 PRO Ref RG11 Piece 2131 Folio 96 Page 8
Marr Age Sex Birthplace
Henry ALLENS M 50 M Curry Mallett, Somerset, England
Rel: Head
Occ: Labourer (Ag)
Sarah ALLENS M 42 F Broadhembury, Devon, England
Rel: Wife
Occ: Labourer Wife
Sarah ALLENS 15 F Sheldon, Devon, England
Rel: Daughter
Occ: Labourer Daughter
John MANLEY 11 M Hemyock, Devon, England
Rel: Stepson
Occ: Labourer Son
William ALLENS 7 M Sheldon, Devon, England
Rel: Son
Occ: Scholar
Emily ALLENS 5 F Sheldon, Devon, England
Rel: Daughter
Occ: Scholar
Lousia ALLENS 3 F Dunkeswell, Devon, England
Rel: Daughter
Occ: Scholar
Florence ALLENS 1 F Dunkeswell, Devon, England
Rel: Daughter
From the census information it looks as though both the mother and step father were married before. I hope this additional information helps you in your search
Regards Jen
