Tim: "I mean in practical terms, not administrative - rezones in terms of use and "value" in the mind of the State, rezones in terms of taxable value."

The mind of the state would be irrelevant if the taxable value were set according to the systematic use of market data. Even if the system were imperfect, it would still be a better option than maintaining the current system of landholders enjoying an extensive state granted privilege. To me, maintaining a system of privilege because it might not be possible to resolve it perfectly is a little like saying we shouldn't try to catch murderers because we won't be able to catch them all, so it would be unfair to the ones we do catch.

"you risk introducing a greater evil, AFAICT, or at best have no idea how things will turn out."

I don't believe it would do, but uncertainty about how things might turn out is not, in my opinion, a good enough reason to maintain the status quo. In order to avoid the problems that would come with instantly zeroing land sale prices, the charge would have to be introduced gradually in any case, which would give plenty of time to iron out problems.

"As I mentioned elsewhere, if you want to talk about enclosures that is one thing, but you are not. Land cleared and developed long before others arrived and then the clearer forced to "compensate" others for the fact that it was cleared..."

If land is being held privately, it has at some point been enclosed. Nobody is suggesting that anybody should pay land rent because they've cleared land anymore than they would be charged more for building a house on the land. The reason that land rent would be levied would be reflect the fact that the holder is enjoying a state-granted monopoly over the land.