Hi Colin,
That does look like a black tailed skimmer to me also.
I can only guess that the dragonfly returning to the same perch was because it's a "favourite" - many species will do this and will return time and again to the same perch. Perhaps it's just at the right height with decent visibility for hunting.
I've not heard of the nymph predation but wouldn't be at all surprised if it did indeed emerge from that stem. Maybe that stem is simply a popular dragonfly thoroughfare
I'm sure some of the more experienced dragonfly peeps can expand.
TobyH