Although looking like a shield bug, this one is a 'Coreidae' - a family of sap-sucking insects. This one is probably Coreus Marginatus, as it has two very tiny horns between its antennae (see close-up photo). Slightly larger than the Green Shield Bug, it is around 15mm. It is known as the Dock Bug, as it often feeds on dock leaves. These were found on the ridge walk, near the Rotunda - a great place to see bugs as there are several paths in and around the vegetation.