Hi Jim
There doesn't appear to be a POTTS/SIMPSON or SLATER/SIMPSON marriage in those lists of Alston marriages does there?
But - now I see...nephew Robert SIMPSON was with George & Jemima POTTS all the while 1851-1871
1851: age 26 - b Alston Cumberland
1861: age 36, b NK Cumberland
1871: age 45, b Alston Moor, Cumberland
Robert wasn't apparently with them in 1841, or at least not listed in the Kelloe, Durham household with Jemima and the first 3 POTTS children - but then again, neither was George POTTS enumerated there, so possibly he was down the pit and got missed? And maybe Robert SIMPSON was with him.......
We can possibly narrow Robert's place of birth though, to Nent Head, which is within the Alston Moor parish.....
http://www.genuki.org.uk/big/eng/CUL/Alston/"ALSTON, (or Alston Moor or Aldstone), a parish and market town in Leath ward, in the county of Cumberland..........It contains the curacies of Garrigill and Nenthead, and the hamlets of Clarghyll, Nentsbury, Leadgate, and Nenthall. The town stands on the slope of a hill, on the right bank of the south Tyne, near the confluence of the Nent with that river." Since: in 1881, it seems Robert is living with his cousin, William POTTS age 27 ( George & Jemima's youngest ) and his (William's ) wife & children:
Cousin: Robert SIMPSON 56, unm, Superintendant Coal Inspector, b Nent Head, Northumberland (sic).
RG11/ Piece: 4969/ Folio: 19/ Pg 32
So as much as anything, possibly looking for a parish record of his baptism in Nenthead circa 1826. At least that has a time frame, rather than looking for a marriage which may have occurred years and years earlier (if Robert was child born later in a marriage, for example, or if he was born to an unmarried mother)
Cheers
AMBLY