Hi sorry but there seem to have been some technical issues and my replies haven't posted.
Yes, I have a copy of the birth certificate. The date is 27th May 1927 (which I believe is shortly before adoption was formalised in law) which gives her place of birth in Surrey.
I don't think it was a formal adoption because her birth certificate shows her birth Mother's surname, and she kept that surname until her marriage.
Sorry for confusing about the adoptive Mother, She was born in 1917 in Sussex. The adoptive Mother's family are all in Sussex - Brighton and Horsham area.
The birth Mother's family are all in Surrey, mostly around the Chertsey area. I can't find any family link between these two families and the distance between them means it's unlikely that there was any chance they somehow knew each other.
I've looked at 1939 register and have both birth and adoptive Mothers.
The birth Mother married (not my relative's Father) 7 months after giving birth and by 1939 had 4 children and was living in a cottage in a tiny village West of Horsham.
The adoptive Mother was working as a drapers assistant in Brighton, her Mother (then aged 66 and a widow) and brother are at the same address. There is also a 10 year old girl, whose name has no connection to my research and another child whose record is closed. I am considering sending my relative's death certificate to see if this record can be opened, but as I do not know where she was living in 1939, there's no guarantee that this is the correct record.
On the birth Mother's side of the family I've come across a number of instances of children living with other relatives (while the parents were working away in service) and some of them have taken in foundlings – which I know there was a payment for. So what really puzzles me is why this child wasn't taken in by any of her own family, and how did she end up in another county?