553 was (possibly still is) 2 separate contracts.
Monday to Friday every 30 minutes (Regent Centre - Freeman Hospital) contracted by NHS
Monday to Saturday every 60 minutes (Freeman Hospital - Battle Hill) contracted by Nexus
Arriva worked with both contractors to Run the service across Freeman Hospital.
I believe Arriva decided to run the Battle Hill - Freeman Hospital section commercially last year, so not to split service up, as i'm led to believe that they had lost Nexus contract to GCT.
RE: Arriva North East: Upcoming Service Changes