I don't know much about that part of the Nene personally, although if I was going to fish it, I would get an Ordnance Survey map of the area, and look for any stretch with features such as side-streams, bays and backwaters. If you just want a big fish, then an area adjacent to or downstream of a carp holding still water would be an obvious choice, although its origins might be open to speculation. Don't worry about fishing any water below Northampton that doesn't have a reputation for carp, as there are so many of them in that part of the river that you certainly won't be wasting your time.
Why not do what I do, and find a place which looks carpy, and where you just enjoy being there. If there are no other carp anglers then all the better.