I used to live in Ipswich – there was some surprisingly good bits of singletrack to be found. No hills, but loads of narrow trails through woods that could be hit at warp speed due to the flatness.
There was some good stuff to the east of the town – around Foxhall Stadium, and one of my favourite bits of trail from the dip in Foxhall Road down through the woods.
Also some excellent cheeky trails around Orwell country park, but I fear they may have washed into the sea by now, as they were getting visibly closer to the edge on a weekly basis, and that was 8ish years ago.
Tunstall was excellent. I’d also recommend it over Thetford. Many of the best trails are hard to find though – I stumbled on many of them just through exploring – definitely worth trying to hook up with someone who knows the network. As it’s flat, there are no landmarks and it’s quite easy to lose your bearings (don’t rely on a network connection for maps on your smartphone!)