I think you are right, as my William was also born about 1798 and he married Hannah. I have found the birth of a William in 1796 on the IGI so I will check the film at the local LDS and let you know what comes of it. As a matter of interest, I found the following entry in the 1841 Census :-
Grays Thurrock ref. HO107/324 or 322 Address (looks like "Blkshots") Sickels Heath
William Knopp40Ag LabourerBorn In Parish Jane Knopp40Do James14 Eliza12 Mary10 George8 William6 John5
No mention of a child named Jane.
My grandmother was Ellen Knopp and she married William Goode in 1904 then came to Victoria in 1922 after William was killed in WW1.