That death of William (place not specified) seems to have come from an LDS submission in the IGI. It gives William as born in 1839 in Wisbech (no specific date), parents as Thomas MOTT & Mary SMITH, and death date (place not mentioned) as 3 May 1917. Whether the birth and parentage details had/have any basis of fact is quite another matter ...
Incidentally, FreeBMD has births for Owen MOTT, Jun qtr 1838 and Charles MOTT, Dec qtr 1839 - both in Ely.
There's a Pedigree Resource File for William MOTT and Frances DENMAN - it lists Frances's birth details but none for William. The submitter's contact details are listed on FamilySearch.
JAP
PS: You haven't indicated which state the marriage of William MOTT & Frances DENMAN took place in nor when or where William died. Did these events happen in a state where birthplaces, ages and parents' names are included on the certificates?