I'm pretty sure this is early instar Broad-barred White. There were quite a lot of them feeding, fully exposed, in the middle of the afternoon on hawkweed (Hieracium sp.) flowers across the site I was looking at today. I'll rear a few on to check, but well worth keeping an eye open for them on hawkweeds elsewhere.
apologies for the poor quality, but my phone was clearly freaked out by the intensity of the yellow