Sadly, that sounds bout right. Assuming you don't need planning permission to plant trees, PPS5 would have no clout at all.

From the looks of things, a quick read of the text of the 1979 act indicates that if the local county archaeologist was prepared to hassle the secretary of state, currently the lovely Mr Pickles, an agreement of stweardship could be put in place which could, depending on the way the agreement is worded, preclude any planting, even retrospectively in the case of planting done since the act was last amended. I could just be making leaps of wishful thinking here mind you.

There's a lot of concern in some places about the effects of planting near rock art panels, much of which was done before 1979. The problem isn't only the effects of the roots and stuff, but also wht happens if a tree falls, gouging out a big chunk of soil, or what happens during deliberate harvestng, where the industrial machinery used can easily wreck the surroundings.