A tiny moth, having a wingspan of only around 6 mm, with quite a coarsely-scaled appearance.  The larva mines the leaves of Hypericum species such as St John's Wort.

Adult Leafmine

Wherever the larval foodplant, St. John's-wort (Hypericum), occurs. 

The species has two generations, on the wing in May and June, and again in August.

The larvae feed in a mine in a leaf of St. John's-wort (Hypericum), at first in a narrow gallery, later becoming a blotch mine.

Fairly frequent in England and Wales. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Occasional in Leicestershire and Rutland, where it is usually recorded from the leafmines on St John's-wort.

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

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/nepticuloidea/nepticulidae/fomoria/fomoria-septembrella/