Purple-barred Yellow Lythria purpuraria
This day-flying species occurs widely in mainland Europe, but has a doubtful status as a British moth.
There are a couple of records from the late 19th Century, from Perth in Scotland, and in York, but since then there has been no sign of the species in the British Isles.
Abroad, the species flies from April to June with a second generation from July to September.
The larvae feed on prostrate knotweed (Polygonum aviculare).