Noticing that the taller sallow was looking a bit shredded with some sort of feeding damage, I quickly found the culprit was the fat caterpillar above. It is a copper underwing, Amphipyra pyramidea. It can be distinguished from its lookalike Svensson's copper underwing by the break in the white line on its side towards the front end (lower left in the picture) and the fact that it has a yellow rather than a red tip to its rear hump.
It is a new record for the window box and must have fed up very quickly as I have not noticed it until today. The moth flies later in the year and the species overwinters as an egg, indicating that it must have survived in this condition in the window box for six or seven months.