Wingspan 5 to 6 mm. The adults are similar to E. heringi  but otherwise quite distinctive, with two creamy spots and a creamy basal patch on a violet-tinged blackish forewing.

The larvae mine Oak leaves, initially in a gallery following a vein, then creating a triangular blotch between vein and midrib. It can be distinguished from the similar mines of E. heringi by the presence of a slit or lose triangular flap in the lower epidermis which allows frass and water to pass.  The larva has a dark head, and continues to feed in 'green islands' in fallen oak leaves. 

Other Ectoedemia on oak

Adult Leafmine

Photograph underside of leafmine to see open slit or flap fhat allows frass to fall out.

Around Oak.

Adult: May to July.

Commonest in south-east England, the distribution expands north and westwards. In the Butterfly Conservation’s Microlepidoptera Report 2011 this species was classified as common.

Increasingly well recorded in Leicestershire and Rutland, usually from leafmines on Oak.

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

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/nepticuloidea/nepticulidae/ectoedemia/ectoedemia-subbimaculella/