Confidence: It's likely to be this, but I can't be certain.
Notes: Just to expand on the taxonomic issues. Continental European authorities say that E. bistortata is just a form of E. crepuscularia. UK authorities nothing treat them as separate species noting that E. bistortata is two brooded whereas E. crepuscularia is single brooded (even in captivity) and that they seem to have different phenologies.
In the British Isles separation of these (as adults) is done on the basis of flight season. Unfortunately mid-May is a pretty ambiguous period so it could be either for my money.