Confidence: It might be this.
Notes: I'm afraid I can't give an identification to species, but can make some suggestions.
This has to belong either to genus Staphilinus or Platydracus: the two are quite similar. The characters used to seperate them are head shape and shiny patches on the end of the abdomen. I can't decide on head shape from your picture: in Platydracus the sides appear straighter and taper in more obviously towards the had, and the widest point is right at the back of the head. In Staphilinus the head appears like a rounded-off square, so is widest nearer the middle (seen in dorsal view). The other character is the shiny patches near the tip of the abdomen. Because these are made of closely-spaced minute hairs, the appearance changes with angle of light, so interpretation from photograph is also rather risky. However, I'd say that yours has paired patches, putting it with Staphilinus where Platydracus has the rear-edges of the last two segments covered entirely. Staphilinus has 3 species, S. erythropterus, S. dimidiaticornis and S. caesareus. S. caesareus has entirely pale antennae or nearly so, and a restricted distribution, known mostly from New Forest. The distinction between the remaining two depends on whether the scutellum, the small triangle where the elytra and thorax meet, between the bases of the elytra, is "covered with conspicuous yellow pubescence" or "mostly covered with black pubescence (sometimes fringed with yellow at base or sides)" [Lott & Anderson, The Staphylinidae Pts 7 & 8, Handbooks for the Identification of British Insects]. In practice, no matter how I angle my specimens of S. dimidiaticornis the scutellum remains resolutely black. On my one specimen of S.erythropterus the scutellum mostly appears grey-silver, only showing the yellow pubesence if the angle of light and viewing is spot-on. On yours I THINK it is like S.erythropterus. This species appears more slender than S. dimidiaticornis - as yours does - though this could only be regarded as a supporting character. Lott & Anderson give "Open environments on poor damp siols, occasionally in mires". S. dimidiaticornis is a common species in my area, though nationally restricted. Perhaps this will evoke a response from our reticent coleopterist colleagues!