Does anyone know

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Precisely, I'd guess it's at Warborough Farm:

http://www.multimap.com/map/browse.cgi?client=public&X=437500&Y=185000&width=700&height=400&gride=439834.258427899&gridn=187938.669141548&srec=0&coordsys=gb&db=freegaz&addr1=&addr2=&addr3=&pc=&advanced=&local=&localinfosel=&kw=&inmap=&table=&ovtype=&keepicon=true&zm=0&scale=50000&multimap.x=401&multimap.y=174

(sorry for the crap url)

Well if it's that, the land completely surrounds the fort, was subject to a successful appeal against being designated open access (hmmmmm!) and is subject to a Defra stewardship scheme so the farmer is paid to ensure the archaeology is protected (hmmmmm!)