I thought this striking bug was a type of ladybird, but it turns out that it is another one of those instars. It’s the first instar of the bronze shield bug, which due to its bronze colouring is quite difficult to spot. This species is widespread but not common.
The nymphs feed on plant sap and insects and are partial to caterpillars. New adults normally emerge from July, overwinter as adults, and then mate the following spring. So, it seems a bit late in the summer for a first instar, as we are told that there is only one generation per year.