Comparing the illustrations of HBW, I think it would have to be either V. obtusa or interjecta however I wouldn't be certain about which of those two.
I would rule out the other Paradise-whydah species due to the fact that this bird has a very dark nape, and the other species have much paler napes.
After years of not knowing I've gotten confirmation that this animal was apparently a Sahel Paradise-Whydah. Unfortunately it is not longer present at the zoo, though the species is around at a couple of other places.
