parasitoid hymenopteran ovipositing into maggots (Diptera), dead fox, Bredon Hill

I saw a number of the 'flies' as pictured walking over the maggots, as if they spend many hours ovipositing (inserting eggs into maggots). The maggots are feeding on a dead fox in a field on Bredon Hill, Worcestershire, England, UK. They are are probably of the dipterous genera Calliphora&/or Lucilia - going by the appearance of the flies I saw around the corpse; otherwise Sarcophagidae, probably Sarcophagia, is the other possibly family. The maggots are mostly aligned head downwards (into the food); the orange-brown circular dots on the tail end of each maggot are spiracles for breathing.