X12 workings are slightly different on Saturdays, with one of Durham's weekday boards worked by Stockton on Saturdays while Durham work one of Stockton's weekday boards on Saturdays,
To clarifiy:
Stockton work the 0725 from Middlesbrough on a Saturday (Durham work the 0715 weekday version) finishing with the 1735 to Durham (1740)
Durham work the 0720 from Durham to Middlesbrough which Stockton work during the week at 0705
EDIT: I'm however not sure what works the 1810 from Middlesbrough to Durham on a Saturday as it works off a 17 board during the week
RE: Arriva North East: Latest News & Discussion - July 2015