Before the others try to kill you in there ever friendly way, Have you tried the search section, then have a look at this site
http://norwich4x4ownersclub.com and finaly dont trust the maps as they can always be out dated, If you search green lanes on here im sure you will find a thread about how to find and use the lanes, Im from the sunrise coast so the only lanes I know up your way would be Thetford and the ones near King Lynn and along the coast bit but I only know of them not where they are ect.