Unknown caterpillar in tall grass near river

Unknown caterpillar in tall grass near river

Observation - Unknown caterpillar in tall grass near river - UK and Ireland. Description: Black and orange caterpillar found on tall grass in a field near the river Avon (Kingston, Hants). Looked to be spinning a silk cocoon about itself- presumably to then pupate. Roughly 30mm length?

Black and orange caterpillar found on tall grass in a field near the river Avon (Kingston, Hants). Looked to be spinning a silk cocoon about itself- presumably to then pupate. Roughly 30mm length?