Genie,
There are only two marriages recorded in Sussex for a Charlotte Pelham, neither look like the Charlotte you are looking for.
Extract from the Sussex Marriage Index:
Place: Ripe, East Sussex, Date: 11 Oct 1801:
Subject: Charlotte PELHAM, sp
Spouse: Jonathan TOWNSHEND, bach botp (B)
Extract from the Sussex Marriage Index:
Place: Brede, East Sussex, Date: 22 Apr 1810:
Subject: Charlotte PELHAM
Spouse: Elias NEEVES, botp (B)
This looks like the marriage of the parents, Richard & Ann, that you talk about at Boxgrove.
Extract from the Sussex Marriage Index:
Place: Boxgrove, West Sussex, Date: 23 Apr 1810:
Subject: Richard PELHAM
Spouse: Ann ABARRA
I wouldn't put too much credence on pedigree sources from the LDS. Start looking afresh. Hope this helps. Get back to me if you need anything else regarding marriages, there seems to be plenty of Pelham's in Sussex (249 marriages) so it looks like you are in the right place,
Jane