I was just looking at that map, tring to figure out if theres an area I know of halfway between us, but theres not, though...There are a few areas I do know of:
North east of Downham Market the back of the village of Marham, (on Hoggs drove I think) theres a off road carpark i think something to do with Anglian water, and a couple of walks lead into decent woodland and farmland.
Just off the the main road A1122 near the A47 (runway end) of RAF Marham there are some woods that run along the main road with a car park, but I'm led to believe it might be a "meeting place" after dark so not somewhere I'd like to hang around late into the day, but the woods are huge and it's easy to dissapear into them.
to the North East of Kingslynn is Roydon common, nature reserve not a camping place and dogs must be ona lead as there are loose wild horses but still nice for a bimble.
North of that is Sandringham again not a camping place but nice for a bimble, because it's a royal house undoubtably plain clothes police/security probably CCCTV will be around so not suitable for camping, but a lovely place to walk.