No indications of illegitimacy that I can see.
As above, John’s father is named Williamson on the marriage cert. and there is a Mary Ann Williamson as a witness (sister possibly).
John was Christened on July 30th 1826 at Stanfield, Norfolk. Father John Wilmerson, mother Hannah.
The only other thing which I wondered about is this:
John had a brother, called Fuller (1834-1912). He served 20yrs in prison for attempting to murder his wife (1881 census is in prison in Portsea). Was a name change brought about by disassociation? I don’t know the date of Fullers trial as to whether dates match up however.