Culprit found
For a few days I've been seeing primroses in the garden with caterpillar damage. Tonight I went out with a torch and found one of the culprits. I'm fairly sure it's the caterpillar of an Angle Shades moth, which I think is quite a generalist feeder on shrubs and herbaceous plants in gardens. The caterpillar does not have any striking markings, unlike the adult moth, which is very distinctive (see extra photo, taken in September).
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