I know I've seen these before but had never actually bothered to identify one.
This is a Black Snail Beetle Silpha atrata that came to the garden moth trap on the 31st of October (and I forgot about).
It's one of the Carrion Beetles (Silphidae) but this is apparently a snail predator rather than a carrion eater. It's elongated head (as well as shiny elytra with distinct ridges) is a give away to its identity, and has evolved (I would guess) to help it get into the shells.
It's a widespread species according to the literature and the NBN map.