re tile roof?

One segment of a pyramid shaped old unfelted roof needs re-tiling. [this side gets most of bad weather]

1. Does it have to be felted under building regs or can I leave the existing batten in place.
2. Do you start at the hips [to be removed on stripping] or from the middle working to the edge or as has been suggested from the right side working to the left side. Which???
Many thanks