Hi Clownpunchers, welcome to our a new hobby and our web site. Welcome also to a few problems that I am attempting to sort out. I don’t really want to duplicate the information you are requesting in this topic. There is a landowner agreement database maintained by our friends at GAGB which I can point you to: http://www.gagb.co.uk/gagb/glad/agreement_view.php?p=69 This should be the sole source of information and there is no point in duplicating it.
However, I am attempting to renegotiate the land agreement for Dartmoor and further information can be found in the Changes to SSSI Rules topic which you have already looked at.
The other very important source of information is the MAGIC map (http://benchmarks.org.uk/magicmapit.php?) which shows the SSSIs, Scheduled Monuments and various types of Nature Reserves, all of which will have special restrictions for placing caches and in some cases they will definitely not be allowed.
If I can get the landowner agreement sorted out, that will solve most of your problems as the majority of the common land within Dartmoor will NOT require any special permission. In the meantime, if you avoid the SSSIs and the “Refused Areas” you mentioned above, look at the MAGIC map and place your caches on common land you should not have any problem getting your caches published by our reviewer.