Onion fly, probably with a bit of white rot as well. It's endemic round here since the last few years, but as I say, it's easy to prevent. It's the young plants which are vulnerable, so fleece them in spring. If you grow overwintering onions, you'll probably find they're unaffected as they're big enough to manage it by the time the fly is active. White rot is another matter, but I imagine it's been around a lot longer.
