We are marking this Garden record as a Most Likely Species, on account of it being apparently widespread except in the West Country where it is said to be scarce. There are about 1000 species of Rove Beetle in Great Britain, many appearing similar to Q.cruentus.
We were unable to get a decent photograph of this beetle, further adding to our high degree of uncertainty about its identification, but managed to capture a little video before it disappeared into dense grass.
Online information suggests, as we have observed here, that this beetle likes to rummage around in leaf litter and dead grass (but also compost bins and trees). It is nocturnal and feeds on other invertebrates.