She is wrong. The land registry title deed is “it”.
The reason she’s raise this now is to try to get you to capitulate because she knows that you’re selling the house and nobody will usually buy it with that boundary dispute.
It depends really on whether you are bothered about this piece of land that she alleges but even if you agreed, then does not accord with what the land registry title deeds says.
However as I said the land registry title deed is “it” and if she believes otherwise, then she needs to come up with some legal premise for why she thinks that some online map company knows more than the Land Registry.
Tell her that you have already taken legal advice and you can show her this reply and suggest that she does the same.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have