This nymph was found under Willow so I suspect it's Kybos cf. butleri but I don't think it'll be possible to take this one any further?
I don't think so. I imagine the food-plant is helpful, but it must be circumstantial rather than definite evidence; I don't know how faithful they are to the host plant. I have found similar nymphs on Alder, which might be K smaragdula, and the (unidentified)adult hoppers do turn up occasionally in my moth trap.