I have not been able to discover a lot about these rather impressive Galls. It seems that they can be caused by insects or by bacteria or fungi and that in particular cases it is not always easy to be sure about the cause,
However, a fellow Blipper does have a book about them and even leads walks in a nature reserve to look at them. I am hopeful that when he sees these he might be inspired to offer a few words of enlightenment.
And if there are others out there with some insight I would be glad to hear about it.