John of Ratcliffe Culey was the son of Thomas, not William.
Thomas Thurman of Ratcliff married Catherine Gregory on 18th October 1729 at Fenny Drayton.
A William and Mary had children, William, Mary and Elizabeth, in Congerstone in 1735, 1737 and 1740 but I can't see a marriage.
David
P.S. - There was a William Ferman and Mary Hacket who married in Markfield in 1833.