I think these are Alder Leaf Beetles. They were at least under a large Alder tree and display all the characteristics.
This was the first time I’ve seen them in the impressive large numbers associated with a fresh emergence. There were hundreds under the one tree, feeding on all sorts of plants. My understanding is that they stay in this phase for about three weeks, then overwinter as adults before emerging again in the spring to start working on the next generation. I think September is quite late for such an emergence though.
Although they are voracious leaf eaters, they are not thought to damage trees in the long run.
Later, 18yo and I headed up to Ilkley to see Mark Watson (comedian) at the King’s Hall. In the strangeness of the times he negotiated coming on after a two minute silence, the national anthem and a cry of God Save the King! over the PA pretty well. We enjoyed the show - 18yo’s first experience of live comedy (other than having me as a Dad, obviously.)