It looks like Dorytomus taeniatus. I think the tree is some kind of willow. It looks like there is a fair number of them up there. The tree is located on a fossilized railway.
No interactions present.
There was another observation of this. In the comments there was a link to a Belgium website that shows one that has been cut open. I can do this and then we will be able to see if there is a pupa in it.
That's the same thought that I had about 5 minutes after I clicked on "Save"!
This is the link i looked at. It shows the gall cut open with the lava.
Mine looks closer to the outside site.
Simon, that very precious link is broken. I think this is right - yes?
Using Google Translate suggests that the discussion on the Danish web site is inconclusive. The larva shown certainly doesn't look like a weevil. One possibility is the larva illustrated was feeding in the gall (an inquiline) but was not the gall causer.
There is little available on web sites about Dorytomus taeniatus galls, although plenty of pictures of the adults. This article refers to a North American species -
(Search for) Colorado Birds April 2011 Vol. 45 No. 2. THE HUNGRY BIRD. Dorytomus Weevil Larvae in Cottonwood Catkins
Note the reference on P.126 thus "This feeding, particularly if it occurs early in the life of a catkin, causes the catkin tip to shrivel and then the entire catkin to arch."
Lat/Lng: 53.26315, -1.22811
OS grid ref: SK515742