It's a mix of owned and maintained (or not maintained) by the local authority or trunk road authority with a small proportion owned by Sustrans and maintained by Sustrans volunteers. Sustrans have undertaken to provide mapping of the entire NCN regardless of ownership of individual route sections.
Some route sections not owned by Sustrans, are inspected by Sustrans volunteers who report the defects they find to the appropriate authority, but these reports are usually ignored.AFAIK.
So it's National network with no one overseeing it, coordinating repair or maintenance , recruiting and managing volunteers. The only thing national about it is a map of a route. So how can anyone begin to even try and fix it? Unless local councils shock horror work together or even more mind-blowing hand the whole lot over to a truly national trust as we have with the canal network. Then in short we are stuffed. Knowing what I know and my past experience with local government empire building Sustrans may as well knit fog.