You've Got No Chance

Want to pin down Mary Francis born 1848 in Briton Ferry.  Well, good luck.
She's at home in 1851 and again in 1861, but not so's you'd know as she's down as Margaret (the name of her deceased sister).
In 1871, she's with her father up in Bishop Auckland, County Durham, but they are both listed as born Glamorganshire, making her one of 11 born in about the right period.
After she marries (1875) her age is listed wrongly in all censuses up until the end.  I only found the marriage as we had a garbled version of her married name from the family bible.  And no mention in either record of the fact she'd been married before up in Durham.

Want to find this lady, another one of my Grandpa's fearsome great-aunties, in the records?  Sorry: you've got no chance!