|Tachinidae (Eurithia sp. ?)|
The larvae are generally internal parasites of other insects, especially moth caterpillars. If I am right about this one's identity, then species of this genus parasitise quite a range of larger moths, especially Noctuids and Arctids.
EDIT: Chris Raper of the Tachinid Recording Scheme confirmed the generic identification, "Yes, that's certainly a Eurithia sp. but it's hard to go further because we can't see the 5th tergite very well."