The larvae of the sawfly Scolioneura betuleti mine the leaves of Birch (Betula) species and those of Green Alder (Alnus viridis). 

Mines leaves in May - June and July to September.  The earlier generation was thought to be a different species, Scolioneura vicina, but it is now thought that they are the same species. 

The leaf mines are large and transparent, without much frass. The larvae have dark markings on the pronotum, two dark spots on the thorax underneath, black legs and dark crescent shaped marks along the ventral side of the abdomen.  The larvae lose pigmentation in the pre-pupal stage.

several other sawflies mine Birch leaves

Adult Leafmine (when host plant is identified)

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

Tenanted leaf mines occur between August and October.

Larval mines may be encountered when the larval foodplants are in leaf.

Widespread in Britain.

Probably frequent in Leicestershire and Rutland 

http://www.leafmines.co.uk/html/Hymenoptera/S.betuleti.htm

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

https://www.sawflies.org.uk/scolioneura-betuleti/