Altica species

There are seven species of Altica (Chrysomelidae) in Britain, and they look pretty much the same externally, so they generally need to be dissected!
Sometimes, a very clear host plant association can help... But only if it's sure that the whole beetle population was sitting on one particular plant. The adults often jump around and land on other plants, that are not their actual (larval) host plant, they sometimes even nibble on them.

(Michael Geiser)