Confidence: I'm as sure as I can be.
Notes: The 3 species involved are this, P. purpuralis and P. ostrinalis. In P. purpuralis the mark on the dorsum of the forewing is prominent and always has the side nearer the base of the wing rounded. P. aurata can have quite a lot of pale marking on the forewing and this is the most common source of confusion (as it has been for me). P. ostrinalis generally has the discal and dorsal spots joined, often black-edged. It is true that to be sure, the underside of the forewing should be checked. In P. purpuralis the pale subterminal line runs parallel to the termen and fades out before it reaches the costa. In P. ostrinalis it curves and meets the postmedian line at the costa. I hope that isn't too confusing. If it is, check out the entries on the Hants Moths site.