When I first saw these bugs I thought they were different species, but they are in fact all Green Shield Bugs. Most insects develop in stages, these bugs have several stages from nymph to adult, these stages are called 'instars'. The one below is probably the first or second instar.

This one is the forth instar, the penultimate stage before it becomes an adult, when it will lose the black colouration. They are named after their shield shape. Both of the young ones were found in September (quite late in the year) in the shrubbery flower beds.

This one is the fifth instar, the final stage before it becomes an adult, with all over green colouration. It's not easy to spot, due to its colour. As you can see in the photo, at first glance it looks like part of the leaf. They are just over a centimetre long, and stand quite still on leaves in the summer. This one was found on the ridge walk, near the Rotunda. As the year progresses, this bug will turn from green to bronze before it hibernates.

This one is the adult, in the spring. It is part green and part bronze. These two are getting ready for the breeding season.