It's worth noting (Rootschatters) that Sarah was the sister whose child was christened the same day as Augustus. Augustus was christened first with Edward and Margaret as his parents and Sarah's child second.
From a tree I saw there was only four years between Augustus and the previous child of Edward and Margaret. Margaret, going by that tree, would have been around 45. Her age is possible and although the other children were regular until that small gap, her fertility would have been declining so it's not impossible that he was a little late or a miscarriage/stillbirth in between. The tree also indicates most of the other children died in infancy.
The outside possibility of Augustus being Sarah's was why I looked with no surname, in case he was registered under his father's (one of mine pretended to be married to the father at the time of birth). No joy. I looked for "Male JONES" too and although one appears in 1858 on FreeBMD I couldn't find it on GRO. Also wondering if he didn't get registered. Thinking about it, I didn't look to see if the other children of Edward and Margaret were registered...