Wingspan 10 to 16 mm. The adults are difficult to distinguish from Eriocrania sangii without reference to the genitalia structure.

All Eriocraniidae mines have spaghetti-like frass in long threads. The larvae live a blotch mine at the leaf margin which over-runs the short gallery at the start of the mine.  The mature larvae are white with pale brown head.  Early instars have a dark pronotum and dark ventral spots.  Short lateral projections on 1st abdominal segment may be visible on mature larvae.

other Eriocrania on Birch. Early instars of Eriocrania (Heringocrania) unimacullella larvae also have ventral spots (ref.: Bladmineerders.nl). 

Adult Leafmine with 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: photos are required showing the mine, backlit to show frass, and the mature larva photographed outside the mine; note host species.

Where Birch is present.

The adults fly in March and April, especially in sunshine.

The larva mines in a Birch leaf, forming a large blotch, from March to May.

The commonest and most widespread of the Eriocrania species that feed on birch, occurring throughout most of Britain. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Infrequent, though possibly under recorded in Leicestershire and Rutland.

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

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