Nest Building?

For a couple of weeks now, a pair of Magpies have been bringing twigs
to this tree in our garden and stuffing them, with some difficulty, into the tangle of branches near the top .  I assume they are trying to build a nest, but over the two weeks there seems to have been very little progress.  I shall continue to keep an eye on them.

This isn't a great photograph, but it can be difficult to catch the bird visible between the branches - and even my longest lens isn't long enough for this work.

I have now posted an album on Flickr of the photographs I took yesterday at Battersea Power Station.