On the prowl

This little beast is an ichneumon wasp or ichneumonid.  It is a parasitic wasp and was searching around the wall, looking for a suitable host in which to lay its eggs.

The Natural History Museum's Beginner's guide to identifying British ichneumonids says there are 2500 species in this country and that they are notoriously difficult to identify.  All I know about this one is that it is not one of the commonest 38 species shown in the guide.