Length 6 to 7 mm.  The forewings are whitish/greyish,veins that have alternate light/dark sections.  The broad pale band down the pronotum, the scutellum markings of 3 triangles and a horseshoe shape, and the pattern of spots on the face can be diagnostic.  The genae ('cheeks') are pubescent - the male has longer, denser hairs. 

Other Idiocerinae on sallows and willows

Detailed examination of the specimen is needed to identify this species.  Take photographs of top, side and face, and note host plant

Willows (Salix alba and S. fragilis).  May overwinter on evergreens.

Adult: August to September sometimes later

A previously scarce species found only in southern England, it now appears to be well established and spreading its range. (ref British Bugs webpage for the species; link below)

Occasional

https://www.britishbugs.org.uk/homoptera/Cicadellidae/Idiocerus_herrichi.html