Chartoscirta cincta

3.5 to 4 mm. Distinguished by the antennae, which are of uniform thickness, with the 2nd segment dark. The wing membrane is long (half the length of the corium) and the upper surface lacks long hairs.

Chartoscirta cocksii (distinguished by the antennae - 3rd and 4th segments thicker than the 1st and 2nd; second segment is pale except for the base).

Antennae of uniform thickness, 2nd segment dark. Wing membrane is long (half the length of the corium). Upper surface lacks long hairs.

Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed.

In marshes or at the margins of rivers and lakes.

All year. 

Common and widespread in Britain.

Few records from Leicestershire and Rutland, though possibly under-recorded. 

https://www.britishbugs.org.uk/heteroptera/Saldidae/Chartoscirta_cincta.html