At this point I decided to look at my copy of Larger Moths of Warwickshire and found a photograph of one under the heading "vagrant moths". Eh? A quick look at the text and it turns out that there are only four Warwickshire records. The county status is described as "uncertain" and "suspected vagrant".
I need to tell someone.
PS Thank you to Kevin for his comments about Scarlet Tiger. He has seen them in his garden in Stratford, so perhaps they are not as rare as they were in 2006, when the Warwickshire moth book was published. Insects seem to be capable of incredibly fast status changes, e.g. Tree Bumblebee and Roesell's Bush Cricket. Global warming in action.