Today I took down the box to examine the contents. There were a variety of insects in it, and five or six spiders. There are only two eggs, so presumably a chick died, and the parents removed the body.
Here is a photo of the two eggs - it really is amazing to see how tiny they are. They are about 1.5cm long and 1cm wide. The depth of the nest material at the back is about 7cm, at the front where it has been trampled down it is about 4cm and the bottom of the nest cup has only about 1cm of "bedding". Next year we shall put some measuring marks on one of the walls in order to enable us to estimate these things without waiting for the chicks to fledge.