Careful! Not all Fen Stanton baptisms have been extracted onto the IGI, no males for a start, so what you see are member submissions, which can be highly unreliable.
However they have been extracted onto the BVRI and these 5 entries between 1805 and 1821 are OK, but there's still no Joshua born c1816. The pattern seems unusual though, for a couple who married in 1799 - big gap until the first baptism in 1805, then 1809-1811-1818-1821
It doesn't appear as though there were any other Harradine couples having children in Fen Stanton at around 1816 - David and Sarah who married in 1805 baptised one daughter in 1806 and then apparently disappeared as I can't see burials for either, or any other children baptised, nor can I see them in a census. But I don't think you can necessarily rule them out.
Do you have the witnesses from the 1837 marriage to see if they provide any clues?
David