This answer is just on spec - I hope someone else can contribute.
I wonder if Olive can be so lacking in olive tones?
We can't see the crown or the throat, which would be definitive, but I wonder about Blue-throated Brown Sunbird? It's in the location according to the books.
Some online photos fit well - for example, if you imagine
this one (the male third down on the right panel) turned to the angle of your bird, then I think they might look like a fit. It doesn't seem to be always as dark as the illustration in 'Birds of East Africa'. Your bird also seems to have a hint of white where the pectoral tufts would be (the book says they would be pale yellow, but in the illustration, they are white) - or is this just my imagination?
Anyway, this is just me throwing something out to encourage others. Maybe Olive Sunbird can completely lack olive?
Edit: this was written before I saw your other thread specifically about Blue-throated Brown, which gives me more confidence that this ID might be right.