lfh,
Just a couple of pointers for you....
The marriage laws for England/Wales decreed that the only legal way to marry was in a Parish Church ( i.e. an Anglican church). So the choice for Presbyterians ( Methodists, Roman catholics etc) was to marry in an Anglican Church, not marry or go out of England/Wales to marry, e.g. Scotland.
I am assuming you have looked for their marriage in Belford Parish and it's neighbouring parishes !
THey could have popped over the border into Scotland and married in a "regular" Presbyterian church or had a "quickie" , called "Irregular" or "Border" marriage, at for example Coldstream, or Lamberton.
Although the couple used Belford Presbyterian church, do you know if they lived in Belford village or Belford Parish or beyond Belford Parish ( they may have travelled from a Parish that did contain a Presbyterian church ?
The law was changed from 1837 to allow all to marry in "their" own church (or in a Register Office)
William 60, Rachel 55 on 1841 census. The 1841 census enumerators were instructed to round down ages of those aged 15 yrs and over, to the nearest unit of five yrs. So if William had declared his age as 64, 63, 62, 61, or 60, enumerator would have just recorded 60 ! and so Rachel could have been anywhere from 59 to 55.
(The page ref for the couple on the 1841 was 848/2 37-17)
Michael