Knot Grass Moth
Thanks to its orange and white spots and long protruding hairs, the caterpillar of the knot grass moth (Acronicta rumicis) is hard to overlook. This is also because, unlike the adult moth, it is diurnal.
The colouration of the caterpillar is variable. The ground colour can be orange, dark brown to black. A nice collection of pictures can be found here.
The caterpillars of the knot grass moth like to eat dock and the closely related rhubarb. However, they can also be found on many other plant species. They even do not seem to disdain poisonous spurge plants.
Although it feeds on many plants that also grow in my garden, I have so far only encountered this caterpillar on a cycle path and a wall.