The thing is with the 74 and 684 is that they serve a fair few communities on their way to Hexham. If the loadings are indeed full when departing Newcastle, then I'd imagine its because people are travelling to Wylam, Ovingham, Stamfordham or Matfen rather than Hexham. I may be wrong though.
According to GNE's website there are going to be four buses an hour operating between Newcastle, Corbridge and Hexahm. That would seem to suggest that the X84/5 are operating at a combined half-hour frequency.
RE: Go North East