What I would do after or before Arriva made the changes is:If you made the 23 any longer than it currently is you'd need to add an extra bus to the PVR.
service 23 same route but diverted to go into Dalton Park replacing service 24 ( or new 22).
service 24 (or new service 22) terminating at Peterlee but then once an hour continue to Sunderland as X21 providing a 30 minute service X21, timetable would have to be adjusted to fit the 30 minute frequency for X21.
so to Sunderland their will be service 23 every 30 minutes going the full route and all stops and their will be a X21 every 30 minutes to Sunderland aswell as express
Also I'm slightly confused would there still be 2 buses an hour from Durham to Hartlepool via Peterlee because when I've seen the 22 heading to Hartlepool has a very good amount of passengers on board.