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Certainly not a Dark Green Frit. For one thing the wing shape is completely wrong.
Looks like a female silver-washed to me. Michael, could you explain why you have discounted this common species?
I've put up silver-washed just to see if anyone agrees, but will agree to cardinal if Michael is certain as I am far less familiar with the species than he is.
I've also emailed the Stratford butterfly farm to see if they have any cardinals in captivity.
I await news from the butterfly farm with interest. Of course I considered the Silver-washed, but it doesn't look right to me. Silver=washed may be relatively common, but not in corn fields. This looks like a lost butterfly. I can only suggest you take a look at Lewington's illustration of a female Cardinal in the Collins field guide.
Stratford Butterfly Farm have replied saying that they haven't had this species "for many years". I don't have a copy of the Collins illustrated guide but their photographic guide shows an elongated wing tip for the cardinal which I don't see in this individual.
Further comment from Richard of Stratford Butterfly Farm:
Could do with a look at the underside but it looks very like a female silver washed fritillary. These are flying locally at the moment.
It would be nice to be able to sort this out because if it is a cardinal and is not an escape it is a very significant record! John, I don't suppose you took any more photos did you?
Nothing that shows the underside I am afraid. I am happy with it being a silver washed fritillary. It was fairly early in the morning and it might have come out of the nearby wood to warm up.
Lat/Lng: 52.20318, -1.8158291666667
OS grid ref: SP126561
Arable field alongside a wood