Single gall ( I assume this is what it is) found on leaf.
No interactions present.
To have a fighting chance of identifying the gall causer it is pretty important to know what plant is involved.
It's on a hazel tree.
Do you have a picture of the whole leaf? There are no galls like the one shown here on Hazel.
In response to comment received, I went back to have another look at this tree. As suggested, it is not a hazel: could it be some sort of elm? (most of the English elms had gone before I learned to identify them). I've added a whole leaf photo and one showing the leaf configuration with very its short stem. The leaves are stiffer and more sand papery than those of a hazel. The shrub is about 10ft tall, and has been chopped back repeatedly (not by me!) as part of a hedge so has no discernible shape.
Several of the leaves are now sickly yellow and crumpled looking. Is this anything to do with the gall (which now has a little 'doorway' chewed in one side).
Lat/Lng: 54.07768, -0.84853
OS grid ref: SE754652