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You are right Jorge, it is a Ulidiidae.
It reminds me of an Euxesta, but apparently the only thing similar having been recorded on the outskirts of Africa is the genus Pseudeuxesta from the Seychelles. So that´s unlikely. Than the only thing remaining is the Acrosticta apicalis, which is the only species recorded from Africa and with a circumtropic distribution. I know something similar to or perhaps the real A. apicalis from South America and bith are similar, however the abdomen of your specimen seem rather elongate and slender.
So in short a preliminary id woould be Acrosticta cf apicalis, based on the catalogue of afrotropical flies published in 1980. I haven't looked for more recent literature (yet). Lets see what Valery has to say about it.
You haven´t by any chance colleted the specimen Will, have you?

Cheers,
John