Hi
Well, no luck yet, but looking at the census pages, we have quite a range of dates for John's birth:
1841 - 1797-1801 (age 40 to 44 yrs, rounded down to 40)
1851 - 1802 (age 49)
1861 - 1803 (age 58)
1871 - 1798 (age 73)
1873 (death) - 1798 (age 75)
Not unusual for the time, but it does broaden the range to search.
I've looked at all the Parish registers I have access to with no luck, so you will need to try others only available on microfiche (from Kent FHS), such as Nettlestead, West Peckham, West Malling etc.
West Peckham might be a good one, as on the 1851 census Mary (born abt 1824), now married to Thomas Foreman (marr in 1844) gives that as her birth place. As she was the eldest, it appears, that might be a good place to look for John/Jane's marriage. We can be pretty sure this is Mary Sunnuck, as brother Thomas is lodging in the same house.
I may also have found another child for John/Jane. On
http://www.west-malling.co.uk/V2/baptisms.php5 they list baptisms for Henry, Grace Caroline, and Ellen. Ellen wasn't on your list.