I thought about that Jimmi - the quick in/out from Pilgrim St, should assist with reliability and time keeping.
It is near Eldon Square, Grey St, Northumberland St and saves people in those areas, walking up to Eldon Square.
Passengers from Eldon Square to Durham, aren't going to be big numbers, but have the quicker X21 - over the X2 (I imagine those travelling to/from Durham aren't loyal to a particular service, unless they hold a pass).
The 22, would offer the slower service - with layover time at Chester, to assist with reliability or time lost en route.
RE: Go North East: Service Suggestions
'Illegitimis non carborundum'