I noticed an unusually tight cluster of twigs on the willow tree in the garden and checking its development over the summer, it turned into a kind of Witches Broom.  I've never noticed this kind of growth on a willow tree before and thought at first it was just a chance mutation, but thanks to the RHS website I now know it is caused by a specific gall mite called Stenacris triradiatus (species #340).  Comparing photos from June and September, you can see how the witches broom has developed with densely packed under-sized leaves and then the affected part of the tree prematurely died back.  There are only two growths on the tree, so happily its not a big deal in terms of the health of the tree itself.