Wingspan 7 to 9 mm. Phyllonorycter viminiella has much less distinct markings than most other Phyllonorycter species. The larva mines the leaves of various Willow species, causing the leaf edge to fold over a tentiform mine with many creases on the under-surface.  To be certain of ID, check the pupa; this is in a cocoon and has 2 pairs of spines on the cremaster, the inner pair pointing inwards.

Several other Phyllonorycter mine narrow-leaved willows, including P viminetorum on Osier

Adult Leafmine

Photograph the underside of the mine and note the host species.  

A species generally inhabiting wetland habitats

The adults fly in May and June and again in August.

The main foodplants are the narrow-leaved willows, such as Osier and Crack Willow. 

Distribution of this moth extends throughout England and Wales and into southern Scotland. Elsewhere there are scattered records from Ireland and northern Scotland. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

It appears to be uncommon in Leicestershire and Rutland, where there are few records. L&R Moth Group status = D (rare or rarely recorded).

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

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