Cumbira Wild Life Trust replied... "I suspect the dragonflies you saw are more likely to be golden ringed dragonflies. One of our larger species, they breed in streams with gravel.
White faces darters are astonishingly rare, only found at a handful of sites in England and have quite particular habitat requirements including deep acidic pools wit plenty of a particular species of Sphagnum moss."
Whatever, they were spectacular to observe.