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After checking my specimens I think it is an Actia - perhaps infantula, but I can't see the katepisternum clearly enough to be sure.

Replying to Black's comments on its small size: There are several small species of tachinid in families like Actia , Phytomyptera and Paracraspedothrix. These have correspondingly small hosts, like micro-moth larvae in fungi. Of course, if the host is extra small and doesn't eat well then the resulting tachinid fly is even smaller than normal :D