I don’t see many beetles, perhaps because I don’t spend enough time looking for them, but I could hardly miss these ones, because there were hundreds of them.
I’m not entirely sure what they are and there are many species of leaf beetle that look very similar but, as these were munching on umbellifer leaves – possibly hogweed (I’m not good at identifying umbellifers either!), I think they might be Celery leaf beetles (Phaedon tumidulus).
As well as munching on the correct type of leaf, these beetles were about the right size (tiny, just 3-4mm), and, although they have lines of dimples on the elytron (the wing covers on the abdomen), they don’t have any on the centre of the pronotum (the thorax), which also fits with Phaedon tumidulus. However, I may have got the ID wrong, so do let me know in the comments box if you can positively identify these hungry creatures.