Difficult, always difficult when you get back to pre census era and people have obviously moved.
So, they were settled in Castor by 1789, when first child was born there. Is there any indication that they may have had older children born elsewhere before they moved to Castor? Did Samuel leave a Will at all, mentioning his children? Have you established approx years of birth for Sam and Sarah from ages at burial?
Whether or not they had older children, we know they settled in Castor, and usually the reason for someone to settle in a particular place would be because the breadwinner worked there. So - what occupation does Samuel have? Is ot something that indicates he probably always came from that sort of area, or not?
Traditionally weddings are held in the brides parish, so even if Sam did come from Castor (though your research suggests he wasnt born there?) they probably would have married wherever Sarah was living.
So have you checked parishes around Castor, one by one, starting with those nearest and slowly working outwards? Remembering, as said before, that (depending on the ages) theymay have already been married a few years and loved elsewhere before moving to Castor c1789?
It is really trial amd error going through neighbouring parishes to start with - are there Branstons generally around, or not? If not then likely he came from further afield originally. Indeed so may Sarah, they may have married elsewhere and moved to Castor after marriage.
Isnt Branston a village in Lincolnshire? Maybe the family originally came from there?
