I don't know Newcastle that well.
So basically all agency location at the quayside are walking distance from the central station?
From the central station; Quayside and Quayside west, and also Gateshead and Gateshead Quayside are all about 10mins brisk to at most 15 mins slow walk from there.
Whenever I booked any of the above locations. I drove part of the way and then got the bus to the central station and done this above walk.
If you are going to Newcastle Quayside from the Central Station just turn left and go up from there past the Center for life. If you are going to Gateshead, or Gateshead Quayside, turn right and go down from there to the high level bridge. Walk across it and you are pretty much there for both.