I had a feeling it would be good and sure enough KD Dijkstra identified it yesterday as Notogomphus zernyi, a species he has never seen photographed and appears to be known from just a handful of specimens. Exciting stuff and well done Angela!
The eastern arc mountains of Tanzania are about as exciting as it gets if you are into finding new stuff. Under-surveyed and crammed full of endemics. Notogomphus zernyi though is actually quite widespread, just poorly known and perhaps rather scarce.
Another image of this individual can be found on the Africa Dragonfly photo database.