When I looked at the census for your man I could see he had a large family so I realised that the children at a later age in life fitted in. The combination of an unusual surname and 2 fairly uncommon first names for the parents makes it easier when checking for the baptisms - there seem to be more baptisms on familysearch for the family in non-conformist church registers, this time in Maidstone. I have come across some of my ancestors having some baptisms in Methodist / Wesleyian churches and some in C of E churches in the first half of the 19th century, so not uncommon, I think they were trying out some of the new strains of religion (or the chapels were just closer to where they lived!).
There is this baptism for a Benjamin FURNALL - 29 May 1808 SAINT PETER AND SAINT PAUL, MILTON BY GRAVESEND,KENT,ENGLAND parents: Ann FURNALL, which isn't too far from Dartford
I thought perhaps it may have been a baptism in childhood for him, but as his mother is a single woman (I checked the cityark image) I think it makes it less likely to be your Benjamin, although not impossible, but you would need to see if you could find this other Benjamin in records to eliminate him.