A New House Guest (or two, or three or four?)

I am so excited - I had hung this little birdhouse under the conifer at the bottom of the garden, but it was a bit lost. So I moved it to the fence between us and our neighbour. Pretty soon, the blue tits took an interest in it, but then so did the cats. So I moved it to the hanging basket hook on the patio, and put up a piece of trellis to block off the way for the cats. After a few weeks it seems the blue tits don't mind the move and have taken a new interest in it. I just saw this little one going to and fro with something white (it looks suspiciously like polystyrene) to make a nest. There is some of it caught in the cobweb behind the house. Oh wouldn't it be lovely to have some baby birds nesting in our little birdhouse?