Good old Knettishall Heath. Being only a mile from where I grew up, I use to rag the hell out of the walking trails on my old rigid Shoguns and Konas as a teenager.
You’re not meant to cycle on any of it, but I can’t ever remember getting any grief. The further south you go from the river and car parks the quieter it is and there are some lumpy sections of singletrack. Lots of dogwalkers though, so you need to be watchful.
The eastern triangle between Norwich Lane and the Coney Weston Road used be fun though the trees and the loop trails near the tumuli on the southern side has some reasonable singletrack.
It’s definitely worth exploring for a morning, as there are some bomb hole/gravel pits scattered about. The Peddars Way/Anglers Way are sort of downhill in parts and link up with some other woodlands further north.
I haven’t thought about riding there for years and looking on Streetview brings back some good memories of my yoof!