This is a Louse Fly (Diptera: Hippoboscidae). Swifts and martins commonly carry one species of these as a parasite and bring these back to their nests. Crataerina pallida is the common species associated with these. They may bite humans if they find themselves in amongst your clothes but we cannot sustain them so they are only a short term issue, if at all. They are super flies, but not the most beautiful.