Well I've got a photo copy of the marriage license for john and ann, and i did look it up in the parish register for alwalton. In the parish register is says he is from Hitchin and on the license it says he is from Godmanchester so thats all a bit contradictory, also the joseph and rebecca entry seems to be one of those little errors that crop up in the igi from time to time. I think it was mistranscribed because there was a joseph son of joseph and rebecca born sometime in the 1800s i forget when exactly, and the parish registers for Godmanchester dont have any trace of the entry from 1738.
There was a william thackeray who appeared as a marriage witness in Godmanchester in 1774, i dont know if this was the same one who moved down to Godmanchester in 1786 or not. The marriage was between Nathaniel Stedman and Ann Sansom, Nathaniel Stedman was the son of a relatively well of miller from Rochester! i dont know know who ann sansom was.