My thanks to Roger, who loves a bit of detective work, and Paul: they tell me my scary insect on the chair is a bagworm. Hideous name for a creature as ingenious as this. I think mine should be called the Greater Patio Pouch-Felting Megamoth, pet name Scary Thing (Gerroff!).
Thank you, chaps. The search for knowledge is never-ending. The link to the www.whatsthatbug.com/bagworm fully-functioning website that Roger sent me resolutely refuses to stay live on this post, but it contains lots of pictures of bagworms of all shapes, sizes and variety of cunning disguises; worth a look if you can access it and have a strong constitution.
Oh, and if you get into the site, do note the ads on the left: there really is an outfit that describes itself as the UK's foremost bedbug specialists.....I bet they're good fun at parties.