I am the coordinator for my club's activities in the underground sites in Reigate, which are primarily public tours. Two of these sites are SAMs, one being medieval. EH have always taken a hands off approach to what we do. When we wanted to extend one of the tours into a "wild" part of a sand mine, we asked their advice as we had to clear all the broken glass from the sand before we could allow visitors. The advice was that we should do a trial clearance and determine the age of any finds. If nothing found was older than 1900, they were happy for us to do what we saw as best in order to open up the section. They were happy for us to do the trial and to use whatever local experts we deemed necessary to date items found. There was no interference or bureacracy involved, other than requesting permission to excavate. Had we found anything of any antiquity I dare say they would have provided more advice on what to do, but from my experience I doubt it would have been a show stopper. Although I cannot be certain, I imagine PDMHS have had much the same experience at Magpie. Maybe someone who knows can confirm this. It is all about having a positive "can do" attitude at the end of the day.
If you complain that EH don't want to listen to good advice from you when it comes to the places you know, then isn't it ironic that when they DO want information and advice from us, you treat them in exactly the same way for which you criticise them, by deciding that we shouldn't deal with them. That's not a very constructive way forward, is it?