This turned up in the studio (a giant moth-trap!) It's a Lunar Yellow Underwing, Noctua orbona, as evidenced by its size and the black distal wing markings reaching the margins of the forewing.
It is reasonably uncommon, occuring on heathland and rough grassland.
I plan to set the proper trap again on Thursday night when the weather looks good and I'll be around to check it in the morning. I'm also hoping my new moth book will have arrived by then!