I've never seen anything like it, although I suppose it could be a wasp or an ant with wasp-like colour bands. Found in our garden today (Cambridge, UK). It's about 1.5 cm (half an inch) long.
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It looks like the parasitic wasp, Ichneumon stramentor, as found by my amazing girlfriend, and discussed on Nature Spot (UK). And it's a female to be even more precise:
Description: Length 13 to18 mm. Female - Long tapered abdomen, first half yellow and rear half black, with yellow spot at the tip (both on the top and beneath. Legs have black femora; tibiae are half yellow blending into brown that exceeds to the tarsi. Black thorax (with yellow spot) and black head. Long dark antennae with broad white patch about half way along its length. Wings have a brownish tinge. Male has all dark antennae, no yellow spot to the tip of the abdomen. (emphasis mine)
It can be seen from April to July, commonly in meadows, hedgerows and woodland edge.
The adult wasp lays its eggs inside caterpillars which, after hatching, eats the caterpillar from the inside. Hence the "parasitic" descriptor.