A small (3 to 4 mm) black bodied insect with infuscated wings. The larvae mine the leaves of Birch - see ‘Further info.’ link)

The mine starts in the centre of the leaf.  The larva has brownish marks on head, and short, stubby, brownish feet

Several other sawfly species mine birch leaves

Adult Leafmine

Adult: Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Leaf-mine: Can only be reliably recorded if the larva is present and can be photographed (from above and below).  The host plant must be identified in the comments

Areas where Birch is present.

Adult May to July. Leafmine: late summer.

Although it seems to be widely distributed in England it is rarely encountered or recorded.

Rare in Leicestershire and Rutland.

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

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

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