by Lesley » Tue Aug 09, 2005 6:47 pm
Hi there, Ken
Alan Bardsley's book, First Name Variants, is a brilliant source of answers on questions like this one. (I have the second edition, published by the Federation of Family History Societies in 1996.)
To answer your question, Ann is the 'source' for the name Nanny but Agnus is an entirely different name, derived from Agnes. Of course, there's nothing to prevent a woman calling herself whatever she chooses, whether or not it is 'correct', nor is there anything to prevent the official who is noting the name of the mother making a simple mistake, especially an elderly clerk/vicar who is a little hard of hearing perhaps!
I hope this helps, although I suspect it may deepen the mystery.
Lesley