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Dear Ben,

Your Tachinid seems to be Leskia aurea.
The picture is possibly somewhat misleading: there is a suggestion of a black median vitta on the abdomen, expect that it is not median !
So, I premuse the abdomen actually is quite yellow and the black colouration is an artifact of the flashlight.
Otherwise, the picture is quite consistent with Leskia aurea, which is quite rare actually, mostly found in August. The biology is special: the species attacks caterpillars of Sesiidae, and since those live IN wood, the Tachinid larva needs to enter the treetrunk first before it can meet its host.

Theo Zeegers