Perhaps he was born in Egypt, after the marriage but before they returned to England and had the other two children. Thus explaining why you cant find his birth.
When did his father die? Have you looked for a Will for his father or mother, as that might give more clues about him.
OR, maybe he was born before the marriage and thus was registered under Kurban, only using the name Jones when his mum married, but reverting to Kurban on adulthood.