I have discovered a third pebble prominent, Notodonta ziczac, caterpillar feeding on one of the coppiced sallows in the window box.
The head is on the left hand side and the tail on the right is normally raised (or lowered in this case) perhaps to entice predators to attack the wrong end, or not to attack at all. The caterpillar is still quite small so, if it survives in the window box jungle, there should be more pictures.
The specific name ziczac is said to derive from the German zick-zack (meaning 'zigzag'). It was Latinised to 'ziczac' by Linnaeus when he named the species and is said to refer to the unusual shape of the larva.