Clearing out the loft of our new house and installing new insulation I noticed a couple of small was nests hidden down in some small gaps that aren't usually visible. The one in the picture is about the size of a tennis ball, so a relatively small example (they can be many feet across in extreme cases). Apparently nests are built, used for one year and then abandoned so I presume that mine are dormant (apart from overwintering queens potentially) but that new ones might appear each year.

The nest is made from pulped wood mixed with wasp saliva – essentially papier mâché – and is very light and quite delicate. If you see a wasp in the on a fence post (or your nice teak outdoor table) scratching away at the surface, that's it collecting wood to build its nest. This page has a fascinating explanation of wasps and their nests, complete with some great photos. I thoroughly recommend giving it a look.