Places   (Help with places, streets and buildings in Wigan.)

pieman,
It is quite possible, that the (Back) in the address is the location of the family and not the address. In other words, the living quarters were at the back of the house - in this case, number 2 of Margaret Street.
Due to extreme poverty, people lived in part of somebody's house - sometimes, just one or two rooms at the back of the house and using the back door as entry and exit.