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The cress is growing up through those pansy... so I made the rather foolish mistake of mistaking them for basal leaves. In the cold light of day it's much easier to discern they are two separate plants... but the caterpillars have had all the leaves on the cress, so I don't fancy my chances getting a positive id there - all I can say is the fruits are 15mm long
We all make mistakes,that would make more sense as this Moth larvea feeds on Brassicaceae and cress is of that family,looks like it would probably be Hairy bitter cress by the pods on the picture
Ah, I thought it might have been Hairy bitter cress, but didn't fancy my chances keying out a crucifer without petals or leaves.
I've seen Wavy bitter cress locally and was fairly sure it was a cardamine, I also checked the host plant database with your id of the larvea too to reaffirm that too...
...but i'll never get back the confused half an hour trying to key out pansy-cress! Even looking at the photo now - I can see the stems are not rising from the rosette. Doh!
Lat/Lng: 51.5, -0.2
OS grid ref: TQ2689