This upside-down blue plastic thing is a boat shaped, children's, sand pit; sometimes used as a boat on the pool. It had been put to over-winter out of sight.
Yesterday I lifted it up, and found a leafy nest underneath. I don't know who's living there, but I think it may be Mrs (or Mr) Tiggywinkle.
Hedgehogs are delightful garden creatures, and I like to encourage them. If this is indeed a Hedgehog Winter home, I shall make sure it's there again next Winter too.