Profenusa pygmaea

The larvae of the sawfly Profenusa pygmaea mine the upper surface of the leaves of various species of oak and occasional sweet chestnut, creating a large blister mine - usually with a narrow 'neck' where it begins, but without a real initial corridor. Frass in gradually larger grains scattered in the mine. 

The larva has dark markings on the prothorax and dark spots on the ventral side of the thorax

Adult Leafmine

Leaf mine photographs should be backlit so that the frass patterns and larva can be seen.  The host plant must be identified in the comments

The mines may be found wherever the host oak species occur

The mines may be found when the host oak species are in leaf.

Never more than one larva in a mine. Pupation outside the mine.

Seems to be fairly widespread in Britain.

Frequent in Leicestershire and Rutland

http://www.leafmines.co.uk/html/Hymenoptera/P.pygmaea.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/heterarthrinae/profenusa/profenusa-pygmaea/

https://www.sawflies.org.uk/profenusa-pygmaea/