Elizajb, pm me your email address and I will send you pdfs of the Haltwhistle Presbyterian baptisms.
I can't find a marriage for Mary and William either.
Mary has a son William aged 4, born Haltwhistle, in 1861 census, and his baptism isn't in the Haltwhistle Presbyterian records.
Familysearch has a baptism of a William, 7 Sep 1856, son of William and Mary Hunter, at Haltwhistle Primitive Methodist Church, so thats probably him.
If you just want to confirm Marys parentage (shes very likely the daughter of John Blake auctioneer, but needs confirming), then the marriage cert of Mary and Nicholas Dent should confirm this.
It should give Marys age and status (widow), and place of abode, and her fathers name and occupation. Same for Nicholas. Plus some witnesses. It will give at least as much information on Mary (and probably much more) than you would get from a church marriage record with William.
Did Nicholas, Mary and family emigrate after leaving Archeys Flat Farm in 1863, I can't find them on any census after that?
Alan.