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Well, association of Conopidae with certain flowers does not show in the Dutch and Belgian material I reviewed. (See: http://www.repository.naturalis.nl/document/645910)

They can be found wherever Vespula can be found. Only Leopoldius coronatus and L. diadematus have been bred from Vespula species for sure. It seems likely that the other species also parasitize Vespula. Probably the reason why they're found from the most pristine nature reserves to unattractive backyards in large cities.
Besides L. calceatus, in England, you might want to look out for Leopoldius valvatus. Recently also added to the checklists of Germany, Netherlands and Belgium.

Greetings,

Ectemnius