This is Tachina grossa, one of the largest and possibly the ugliest fly in Britain. It's as large as a bumblebee and spends its days feeding on nectar - harmless enough, unless you happen to be the caterpillar of a butterfly or a moth - in which case, like many other species of tachinid, it will lay its eggs on you. Its larvae burrow through their host's body wall and slowly consume it from the inside. Gross.
Apparently, T.grossa has a particular penchant for large caterpillars - especially those of the oak eggar moth and fox moth
This specimen was spending a sunny afternoon on a hillside at Stanhope in County Durham, feeding on marjoram flowers.
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