Picture-winged fly

I found this female picture-winged fly lurking on a sycamore above a patch of creeping thistle yesterday, so by dint of association and the pattern on the wings I'm guessing that it might be Urophora cardui, which lays its eggs in creeping thistle flower heads. 

That long ovipositor on the tail looks like the right kind of equipment for inserting eggs amongst thistle florets.....

             ..... like these.