You bring up the X12 competing with the 21/X21, I do wonder how GNE's changes to the 21 has affected things, first halving the frequency for people in Birtley wanting to travel beyond CLS and then sending it via the Arnison Centre adding around 5 minutes into the journey time because of the awkward way it has to go into the Arnison Centre (well, that's when they haven't pulled it from serving the stop outside Sainsbury's that is, happened again today with the 21/X20).
That's one thing that concerns me with the Brandon extension, whenever the service is delayed I can envision them dropping passengers off in Brandon then run NIS up to CLS to re-start the service, have noticed X21 do that a few times in West Auckland due to delays, drop off to West Auckland then run light to Bishop Auckland Bus Station to get back onto schedule.
Arriva North East: Upcoming Service Changes