As well as BOAT's there are a number of UCR's that you can legally ride. They too are marked on the maps. If you check the map key at the side of the map, you will see a section under the title "other public access", The UCR's are marked with a series of dots ...... green ones on the OS explorer maps and pink ones on the landranger maps. The Thetford area has several of these and are well worth riding.
Also a good resource is the online Norfolk rights of way map. http://www.countrysideaccessmap.norfolk ... exPage.htm
you can ride the black routes.
There is also a good selection of Byways in and around the Kings Forest but that in Suffolk and therefore not shown on the Norfolk online map. They are however once again clearly marked on the OS maps. The only problem you will encounter in that area is two lanes that are subjet to seasonal closures which run from Oct 31st-April 30th
Hope that helps.