I used Google Images and entered shieldbug instar in an attempt to identify this nymph. I found two close matches - the forest shieldbug and the red-legged shieldbug and spent a while trying to decide between them.
No wonder I couldn't decide - they are alternative names for the same creature!
Here you have it then: a late instar of the forest (or red-legged) shield bug, Pentatoma rufipes (one of two on a sycamore leaf).
As an 'extra' I have added another interesting find - a female scorpion fly (Panorpa communis) The scorpion-like tail is only seen in the male.