Eriocrania salopiella

Wingspan about 10 mm. The adults are golden, marked with purplish, with a distinct pale tornal triangular spot, and a smaller costal spot. The head is covered with yellowish hairs.

All Eriocraniidae leafmines have spaghetti-like frass, in long threads.  The mine starts in the centre of leaf, as a short gallery widening gradually into a blotch.  The larva is white with pale brown head and darker mouthparts (ref. British leafminers), but this can be difficult to see.

Other Eriocrania on Birch.  The mines and larvae of Eriocrania sparrmannella are very similar, but occur later in the season

Adult Leafmine+larva

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.

Leafmine and larva: Only tenanted mines can be recorded. Photograph the mine (backlit) and the larva removed from the mine; note host species.

Areas where Birch occurs.

Single-brooded, flying mainly during April and May.

The larvae mine the leaves of birch Betula during May and June. The species is the latest of the birch-feeders with the exception of Eriocrania sparrmannella, which feeds mainly in July and August. 

Well distributed across mainland Britain, but fairly localised, this species is probably overlooked. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as local.

Rarely recorded in Leicestershire and Rutland, though possibly overlooked.

http://www.leafmines.co.uk/html/Lepidoptera/E.salopiella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/glossata/eriocraniidae/eriocrania/eriocrania-salopiella/