I have Hannah Malson born 1763, who married John Thomas 12 Jun 1791 Newland as daughter of Richard Malson and Susanna Viner.
Looking at Hannah's birth date and that of William c 1788 who married Sarah could it be possible that William was the illegitimate son of Hannah? Possibly named after her brother William born 1774. Just a thought, but it would explain why she was a witness at William and Sarah's wedding!