Shieldbug (Coreus marginatus)
One might think at first sight that this is a beetle but it is in fact a squash bug, so called because the family as a whole are a significant pest of squashes in north America.
This UK member of the family is quite common in the the spring and autumn, especially favouring dock leaves but the later brood are found on blackberries and other fruits as well.
Not an attractive insect it has to be said, its big 'padded' shoulders making it look quite fearsome but it is quite harmless of course being vegetarian.
It is also very variable in shading and can be lighter than the one I photographed and can also be much darker, almost black.