I'm left scratching my head on this one...have read most of the letters...which leave you with a thousand more questions...the handwriting is very need and articulate...the 'mother' never refers to the child by name...only as the little one, and once or twice as my little girl.
I have added her signature, and also how she writes Dear Mrs Wright, to compare what she calls herself. There are very few registered births for a Mona in London, you could go through all the certs, but will this really help?
The way the 'mother' writes in the letter, gives me a feeling that she knows London well...she refers to Kings Cross as Kings x, tells the adoptive mother to stay on the platform, and that she has a room arranged for her...not really language of a country girl...
Arranroots, do you think it worthwhile putting some of the letters up here?
I don't know what else to suggest, or where to look next!