On the 1901 census, you may find that, for example, your great-great grandmother was living with your great-great-grandfather and they had children (if the children were still at home, their names would be given). The great-great grandfather's and great-great grandmother's 'approximate' age would be given, so you can then hopefully apply for birth certificates. You will also at this stage be able to possibly go back to the 1891 and 1881 censuses, and find out who they were living with earlier on in life. This may give details of their parents.

Regards,

Colin