Hello Brian
I don't think Joseph is the son of Thomas,it doesn't stack up.
Joseph would have had his mothers surname unless the marriage Banns had been read by the time of the christening (if there was one) whereby the Rector may have allowed him to be christened Scofield.
If Thomas & Mary had been living together I would expect to see other children prior to the 1813 marriage.
The only expanations now as I see it is that Joseph was the illegitimate son of Mary Bull ( but not Thomas) & was given the Scofield name on marriage.
or
was an illegitimate child of one of the daughters but lived with g/parents after marriage as he may have been rejected by the husband.
I suppose there are lots of scenarios you could come up with from being left on the doorstep by gypsies

to being washed up in a reed bed.
I'll still put my money on the daughters.
Jim