Sarah Ann's birth registration circa 1849/1850 - It's just that, Jules - without us knowing she was in fact born to a woman who had been married in 1846, we would automatically start to wonder if there was more too it - eg: illigitimacy etc and so try to find out more of that possibility in hopes it would lead to a birth perhaps under something other than expected.
I find it somewhat 'amusing' given the problem, that Ann in 1861 and her daughter Sarah right up to 1883, was living as a servant (?) with a man whose occupation was a Registrar of BDM!

You mentioned on the othe forum that Ann had 3 children.
The eldest being Alfred - who also disappeared.
When & where was his birth registered?
Cheers
AMBLY