I came across a small patch of toadflax (also called butter-and-eggs) Linaria vulgaris, with nine of these colourful caterpillars feeding away, all at different stages, with some very tiny instars and some, like this one, quite advanced. Both the moth and the plant are native to Europe. The head, by the way, is at the lower right side ... you can see it is munching away on the leaf.