Hi Lyn ,
I agree with you regarding William being born at Cotton and baptised there 29th August 1744 , base born son of Elizabeth Davey . An IGI entry giving a baptism for William of 23rd November 1744 , son of Samuel Davy and Elizabeth was not found when I checked the registers .
I agree also that Mary baptised 19th January 1766 at Rumburgh was the daughter of Samuel Davy and Elizabeth nee Reynolds - they were married at Rumburgh 15th October 1764 . ( Note however that there were records of the baptisms of children of another Samuel Davy and Elizabeth at Rumburgh prior to the above marriage ).
Are you sure that your Samuel was he baptised at Bungay St Mary 28th October 1733 ? - IGI have a marriage between Samuel Davy and Elizabeth Calver at Bungay Holy Trinity 27th December 1769 - although this may have been confused with another Samuel Davy married to Ann Calver at Walsham le Willows .
Best wishes ,Malcolm