About 7mm long? Distinctive greenish head.
No interactions present.
I think you may be right! I'm no expert, but this could well be a young Froghopper. These fellows are responsible for making lots of frothy bubbly looking stuff on plants in on moors...commonly known as cuckoo spit!
I agree with you that this is Cuckoo-spit bug, but Darren's photos are of the adult bug, and as you say it is the younger nymphs that produce the cuckoo-spit.
Entomologist and biological recorder
Lat/Lng: 51.5, -2.6
OS grid ref: ST6282