(17 Apr 2019, 10:22 am)JP6004 wrote Actually the red zone covers all of the route after openshaw bank top. The network one ticket technically shouldnt be used as outside the nexus area (county durham) however they are generally accepted as it come back into a nexus area. I believe it's only a few stops in Burnmoor

It was clarified that Boundary Houses was the red zone border the other day. 
The section beyond there to Bournmoor/Woodstone Village is within the green zone. Beyond there, the 4 goes back in to the red zone.

I often used a T&W ticket (it was much better value than the Day Rover at the time) on the 21A, despite it heading in and out of Co Durham. 
Unfortunately both the ticket and service were culled.