Thanks to Pam Toothill at Owl Rescue who sent a huge envelope containing an array of fine owl feathers. (Turns out I couldn't be searching for owl feathers at a worse time - they're in breeding season and so don't start to drop their feathers until the end of May… not quite soon enough for me.)
These are Bengal Indian Eagle Owl feathers from a bag of all sorts. They are large (some up to 30cm) and indescribably soft and have a sort of downy layer over them that means when you look at them close up, they appear almost out of focus. (I keep finding my cats just sat with their faces in them, breathing them them in contentedly. And quite loudly. They also do this with bunches of dying flowers on the kitchen table.) Now I appreciate all that I've read about their flight being different from other birds'. As well as feeling like a work of art, each one also looks like it's been hand-painted.