British distribution: Apparently widespread, at least in the south.
A large (relatively) and attractive ichneumon wasp (or "ichneumon fly"), with the black and yellow colouration of many aculeate (stinging) wasps, but instead a member of the large, stingless 'Apocrita Parasitica' group of the Hymenoptera. Presumably it is a Batesian mimic, enjoying a degree of protection by resembling dangerous models.
In view of the huge number, over 3000, of British ichneumon wasps and the lack of easily accessible definitive literature, the identification here has to be somewhat provisional. The naming of these photographs as Amblyteles armatorius matches the current general concept but the possibility of it being a very closely related species remains.
Ichneumon wasps, with a few exceptions, are parasitoids of insects and other arthropods. Most exist in the larval state within the bodies of caterpillars of butterflies and moths. A. armatorius is known as a parasitoid of moths in the family Noctuidae and has also been reported specifically as parasitising the caterpillars of the Emperor Moth (Saturnia pavonia) (Peigler, 1994).
At first glance, A. armatorius might be taken as one of the spider-hunting wasps, but the long antennae are an external characteristic of the Ichneumonidae. As noted above, it is stingless. The female has a shorter, more oval abdomen and lacks the long, protruding ovipositor of most of the ichneumon wasps. The distinctive, bright yellow, dorsal spot at the base of the thorax is seen in both sexes. The great majority of ichneumon wasps are much smaller, black, inconspicuous insects.