Blimey - I'd have thrown that out as straminea!
You may have a point there Dave, it was the strongly curved costa that convinced me initially (typically straight or even slightly emarginate in straminea producing a very narrow-winged look). Given the variable appearance of C. straminea in web photos (not all necessarily identified correctly of course!) I'm happy forget this one. It is worth noting that P. affinita can be very abundant on stands of Sea Aster in the Burry and it may well be worth checking stands in other parts of the county, perhaps next year as it is getting late in the season for affinitana, perhaps another pointer that I may have got this wrong?