"the National Trust and other landowners don't want to bother with "active conservation"......... The sad thing is that there are individuals in these areas who would gladly give up their weekends to clear sites "

The concept isn't foreign to NT, they run an Avebury Guardians scheme for litterpicking etc.

Maybe you could put it to them in the form of a Heritage Journal article? Assuming Heritage Action doesn't tell you to sod off of course. ;)