Hi Bryant, you are uncovering interesting stuff!
No any Dix family from Ampthill are not (at least I believe) the family of James. I did spend time following many of these families and can to the conclusion that they are not related. But of course I could be wrong

However, I did come across a DIX family in Holborn who could be related. And as I mentioned I went through the BMD records looking for DIX members in Edmonton and there were definately some there but I cannot make the link. I should perhaps go back and explore that further.
Perhaps Henrietta is from another child that I haven't found yet. I do know that Emily Caroline was the mother of Frances Lilian born 1881, no father shown.
All of the women were at some time occupied as straw bonnet makers on Fishpool Street where several large straw hat factories were situated. My grandfather had mentioned that to me.
Ok I looked at the 1851 census and cannot find a Mary that fits born in Bucks do I will have to spend more time on it .
I think it's back to the drawing board for me!