A Ruby-tailed Wasp (Chrysis ignita agg.), sometimes also known as a Jewel Wasp. They are a 'cuckoo wasp', laying their eggs in the nest of other solitary bees. The larva emerges to eat the host's eggs. They are allegedly common, but I have only seen one twice. They are only 1cm long so easily missed. This one was in the Great Broad Walk borders @kewgardens yesterday. It was a joy to see so many pollinators there, as it should be.