While doing some decorating work on the house, I spotted a Shieldbug in the conservatory. I couldn't get a name to it and suspected it was a species I hadn't seen. Subsequent checking of the excellent Shieldbugs and Squashbugs of the British Isles by Evans and Edmondson suggests this is the Tortoise Bug Eurygaster testudinaria. It wouldn't be that surprising to see a new Shieldbug, as this was only the eleventh species that I've seen out of thirty-five species covered by the field guide. I've never gone out to specifically look for Shieldbugs, but I have tried to identify those that I've seen.
There are two similar species of Tortoise Bugs. This species and much rarer Scarce Tortoise Bug Eurygaster maura. Identification looks tricky between the two species. One of the features is the outer edges of the head is parallel with the edges of the pronotum on Scarce Tortoise Bug. Additionally, the length of the second segment of the antenna is about twice as long as the third segment on Scarce Tortoise Bug, whereas it is only slightly longer on Tortoise Bug. On this basis and after looking at photos online, I think this is the commoner Tortoise Bug. However, the books suggest that dissection might be the only way to be certain. I prefer to release Insects rather than to kill them to be one hundred percent certain with their identification. Thanks to Phil Saunders for confirmation on the ID.