Ichneumonid wasps account for 10% of all British insect species and are parasitoids, meaning that they lay their eggs in or on another insect and the larvae eat and kill the host species.
Here is another one I spotted near my bee house in the summer. It is a female with a very long ovipositor but I haven't worked out which species. It might be Rhyssa persuasoria, the sabre wasp, but then again it probably isn't.
I found a beautifully illustrated guide to the commoner ichneumonid species from the Natural History Museum which you can download here.