Confidence: It's likely to be this, but I can't be certain.
Notes: Examples of the larva in ukleps match this very closely and the time of year is correct. Rough pasture habitat is right also. Some other Mythimna larva images are similar but not identical. The fine pale lines running down each side of the dorsal area, the dorsal dots and the head markings fit this exactly. The 2nd larva image from the top in the ukleps offerings is particularly good for comparison with this caterpillar. The adult is well recorded in the area throughout the summer.