Looking at the bit of hindwing visible, in light of Dave's recent blog on the subject, isn't that a Clifden Nonpareil?!
EEEK! Catch it!
Ah, yesterday. Nevermind.
Of course it could be just worn. Easier for a RU to lose the black scales than it would be for a Clifden Nonpareil to gain them...
I guess that's the most likely explanation. The forewing tips are a bit worn.
It was definitely a red underwing - we potted it and it was showing off its hindwings!