Hi JohnP,
I am also directly related to this tree - the Shoemaker Smiths of Sharnbrook! I have reached the same sticking point as you but I realise you wrote this in 2006 so hope you still are active on here and see it, or somebody else at least!
Did you by any chance get anywhere with tracking down Joseph Smith's baptism record? I have 3 potential brothers for him ,3 of the brothers share the same trade of Shoe maker, the other is a tailor, but no records for any of their baptisms. Mary is seen living with son Joseph in one census and with son Robert in another. Both share the same trade which connects them and Joseph's daughter is living with Uncle Robert at the same time as Mary was there. I believe the 1797 date to be more realistic for Joseph's birth as the 1841 census giving 1801 is suspiciously full of "rounded up" age numbers by the innumerator.
The only record I can find for Mary is potentially Mary Woodens who married William Smith in 1793 at Sharnbrook. I am trying to trace her baptism record as well and their 4 children. There is a William Woods and Mary who had a daughter 1774 in Felmersham which seems a possible match. But who knows? If anyone can help please respond. Many thanks.