In Britain the genus Cosmoconus is represented by two similar species: Cosmoconus elongator and Cosmoconus ceratophorus. Both species can not be named from photographs. To make a positive identification you have to study them under the microscope. Females are thus not difficult to tell apart, for their saws differ. Males remain hard to tell apart, even under the microscope. We didn't look at them under the microscope, so just the genus name remains. Both species are black, with a largely red abdomen and partly yellow legs. Both measure about 10 mm.
Both Cosmoconus elongator and Cosmoconus ceratophorus live in about the same way. The females deposit her eggs on a Sawfly larva. The egg remains on top of the larva untill it spins itself in a cocoon in order to pupate. At that moment the egg of the wasp hatches and the wasp larva starts eating the host from the outside. Adult wasps appear from May to October.
Probably both species are common in Britain. But that is hard to tell, because not each animal spotted is examined under the microscope. Cosmoconus elongator can be found in a vast area: all of Europe, Russia, including the parts in the Far East, and most of the Middle East.