PCL Travel
Looking more closely at the timetables, I think it's possible that some of the M1 journeys could interwork with other services, the first weekday morning journey to Thropton looks to work onto the 15 and the last journey from Thropton looks to come off the 15, it also seems coincidental that the weekday (excluding Wednesday's) M1 that leaves Morpeth at 10:00 and return journey run during a gap in service on the S1 timetable, unsure what would work the 15:00 from Morpeth and return journey as both the 15/16 and S1 are in operation during these times.