Thaumatopoea pityocampa

The nest of the processional caterpillar. Having just read up about them again, we'll have to see if there are any on our land (this one wasn't) and get rid of them. Last year we sprayed all we could find with hairspray, chopped the branch off into a bucket and put them on a bonfire. It seems to have worked as I've only noticed one nest on our terraces so far.See here.