This is a female Ichneumon sarcitorius, one of around 2,500 species of ichneumon wasp found in the UK. They're all parasitic on other insects and, unlike many parasites, they all kill their hosts. They make up about 10% of UK insects and play an important ecological role in regulating insect populations.
In the case of the Ichneumon sarcitorius, it lays it's eggs on moth caterpillars that the larvae then eat. The adults are harmless and feed on umbellifers like Cow Parsley, Hogweed, Ground Elder and other members of the carrot family. Adults are usually seen from July to October, although this one was seen in Hiddel Brock Wood at the beginning of May.
Whilst some ichneumon wasps lay their eggs on the host like this one, many have ovipositors (a long tube through which the eggs are laid) to insert the eggs into the host. This is not, as it may first appear, a sting.
For those, like me, that like to know the origin of species names, the name ichneumon means 'tracker' although I haven't been able to find an explanation as to why.
If you're interested in learning more about Ichneumon Wasps, the Natural History Museum have a good beginners guide to identifying the most common ones here.