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To be fair to Martin, he did preface his Scatophila key with a warning, that it didn't really work! But you should have got the answer straight away - the face of the male has a concave depression. It is Scatophila caviceps. As you have a male, you can check the genitalia. This is quite a fiddly job, as they are very small. I would recommend taking off the whole abdomen, soak it in potassium hydroxide overnight, then use micropins (mounted in matchsticks) to tweak out the genitalia so that the internal structures can be seen. Don't attempt to do a full dissection - it will not end well!