This Large White was being continually hassled by the Fly in the photo's, until the Butterfly flew off however, the Fly would lie in wait for her to return & when she did, once again hassle/ hitch a ride. It was only the Large White that interested the Fly. Bees & Skippers visited the thistle head, none of them of any interest to the Fly.
I'd be interested to learn what was going on here & of the identification of the Fly - a parasite? I suspected some kind of ant mimic/ scavenger Fly but understood they fed on dung & were not parasitic?
Excellent photos and very interesting behaviour too. The fly is indeed a parasitoid called Phania funesta but Phania are not parasitoids of butterflies - they attack small burrowing shieldbugs from the family Cydnidae (more specifically Legnotus limbosus). Phania belong to a subfamily that are entirely bug parasitoids too, so there isn't a chance that they are interested in the butterfly as a host. Phania do seem to be very attracted to sunny places though and I have often seen them basking in the middle of white flowers so it could be that the fly is getting a bit confused by the large white butterfly.