A wasp that is a parasite on other solitary bees and wasps: a Gasteruption sp..

 My thanks to Karen McCartney for her identification of this wasp as a Gasteruption sp male. 
It was photographed on Hogweed on a bridleway on Wolstonbury Hill on 29 July 2019.

 The females have a long ovipositor and lay eggs in the nests of solitary bees and wasps, where their larvae prey upon the host eggs, larvae and provisions.  Source: https://en.wikipedia.org/wiki/Gasteruptiidae

 The male was alive and well on the hill and a female, below, was found on a windowsill inside my house in Sussex very much dead.

The ovipositor is comprised of three parts in this picture.  She would have found plenty of solitary bees and wasp nests in my garden to attack before becoming trapped in the house.