It was Hecatera bicolorata

This morning I rediscovered the small caterpillar I had spotted yesterday on a photograph of smooth hawksbeard (Crepis capillaris).

It is that of a broad-barred white moth (Hecatera bicolorata).

Particularly satisfying to have recorded a new plant in the window box in spring, seen it flower and run to seed, and to have to discovered a caterpillar feeding on it that I also had not recorded before. 

The larvae of this moth is said to feed mainly on hawksbeards and hawkweeds and its position on the stem with its head thrust into a hole to get at the ripening seeds is characteristic.