Wingspan 7 to 9 mm. The leaf mines caused by the larvae are usually found on the underside and have a typical 'tented' appearance with a crease line across the surface.  The frass is characteristically arranged in a U-shape around the cocoon.

The larvae mine the leaves of Oak, Hornbeam, Beech or Sweet Chestnut, and sometimes on other tree species.  On oak the species is difficult to identify from the mine alone.  The pupa has a distinctive arrangements of hooks at the very end, known as the 'cremaster'.  There is a key and description of these on Bladmineerders.nl here

Other Phyllonorycter

Adult Leafmine (Oak) (Holm Oak/SwChes) (Beech)

Photograph the underside of mine on Holm Oak, and backlit on Sweet Chestnut and Beech.  On other oaks and other tree species, it is necessary to examine the pupa and photograph the cremaster - often this is sticking out of a vacated mine.

Wherever the larval foodplants occur.

The adult moths occur in a number of generations, any time between April and November.

The larvae mine the leaves of Oak, Hornbeam, Beech or Sweet Chestnut, and sometimes on other tree species. The mines can be seen during March and April and again July to October.

Fairly common throughout much of the British Isles, though more so in the south. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Occasional in Leicestershire and Rutland. L&R Moth Group status = C (very scarce resident or rare migrant).

http://www.leafmines.co.uk/html/Lepidoptera/P.messaniella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/ditrysia/gracillarioidea/gracillariidae/lithocolletinae/phyllonorycter/phyllonorycter-messaniella/