Early instar Drinker caterpillar

Early instar Drinker caterpillar

Observation - Early instar Drinker caterpillar. Description: Small (about 15mm) larva, on reed stem amongst nettles and other herbaceous species. I can find lots of things it isn't, but hoping someone can say what it is. Title changed post-identification.

Small (about 15mm) larva, on reed stem amongst nettles and other herbaceous species. I can find lots of things it isn't, but hoping someone can say what it is.
Title changed post-identification.