I am quite sure the censuses detailed in Neale1961s summaries at replies 52 and 53 are the correct ones
Many things still need to be sorted out here
-This still is possible that she never married
-it is possible the 1841 / 1851 census that was found is correct and she moved a town over when she got pregnant and changed her last name to Williams.
-she could of been an escort and had the Children that way? She had 6 kids somehow
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-She could of also been married to a Mr Jones and left him while pregnant with Amy. She then could of changed her last name to Williams like she did in the mentioned Censuses.
-or Mr Williams could of been a real person
-It is also possible that the marriage cert just hasn't been found yet.
-It is possible that she was married in a different country. I have seen that before on my tree.
Either way it is a rather complex family structure.
-I still would like to find Margaret Wilmot's parents it still wouldn't surprise me if her dad was an Edward Wilmot.