Uncle Pip - I think you are completely wrong here.   I rode it last week and its a well surfaced tar road that connects through to the B4343.  I saw no signs indicating any form of private road.
Furthermore, OS data both current and in 2016 list this as the B4574 in the same format throughout its length.
Unless you can provide some compelling evidence as to why I am wrong, I will shortly revert it back.
You have joined OSM, edited the map once, and have not been seen again.  Perhaps this change has been made perhaps for personal reasons, to prevent vehicles using the road, and if so, that is not an acceptable practice.
Rgds, Russ