A leaf affected by Holly Leaf Gall Fly Larva
No interactions present.
Agreeing with Latin binominal, but why is this termed a gall? Surely it is a leaf-miner, starting out as a serpentine mine & finishing as a blotch.
I guess it's a gall because of the reaction/response of the host tissue. But it is an interesting question as not all leaf miners have associated galls, nor do all host tissues react in the way shown here.
Gall - an abnormal colouration, swelling or excressence in the host tissue in response to 'attack' - my definition.
But Wiki does it well http://en.wikipedia.org/wiki/Gall
but, as you imply not all hosts respond in this way.
There is no reference about this being a gall in many of good sources e.g. http://www.nhm.ac.uk/nature-online/species-of-the-day/common-species/phy...
You will find however that if you search for Holly Leaf Gall Fly e.g. www.hbrg.org.uk/TRY/TRY_Pilicis.pdf some authoritative material emerges.
Lat/Lng: 57.023, -6.3474
OS grid ref: NG362008