Having named it so confidently, I have to add the customary rider that it could also be a Dark Dagger. The two related species are so similar that their genitalia, which clasp together when mating, have to be studied closely to distinguish between them. The moth Bible describes the male Grey Dagger male's clasper has having two teeth formed like a pincer for coupling, while the Dark Dagger has three. I haven't the time, skill or dispassion to investigate, I am afraid, and both moths are rated as 'common' (presumably by experts with stouter hearts, as the poor moth has to be anaesthetised, frozen or killed to do the job reliably).
Away from such painful subjects, here is an unmistakeable Powdered Quaker from behind and the side; a delicately-coloured and patterned moth and also quite a decent size.