A medium-sized bug wth an elongated body, associated with wetlands, marsh and damp grassland.  It can occur in a brachypterous form, with very short wings, or in a macropterous form with wings that nearly reach the end of the abdomen; occasionallly intermediates are found.  Nymphs have bright red patches on the abdomen.  It can occur in large aggregations.

Good photographs; note habitat.

Common (ref: British Heteroptera checklist 2021, on British Bugs website).  Until 1893 it was known only from a handful of sites in Kent and Surrey, but has expanded its range hugely and is now widespread in England. 

Frequently recorded

https://www.britishbugs.org.uk/heteroptera/Lygaeidae/ischnodemus_sabuleti.html