brimstone butterflies

Ian, they are fairly unusual!  Larval foodplant is  Alder Buckthorn, and there's not much in Cere!  Arthur Chater's book details plantings at and around Ynys Hir where there are Brimstones seen regularly; closest to you according to the map in the book is square 5952 which Arthur could pinpoint for you.  Otherwise the only plants outside the far north of the county are on the Cardigan by-pass.  Lin Gander, Butts recorder, will be pleased to get the record, contact details on Butt Con website.