Oxalis cornuta, reckoned to be one of the most widespread and difficult to deal with weeds in the world.
Roundup painted on the leaves does kill it. I burn off the top growth with a Flamegun to stop it seeding, but even that does not really kill it.
One of the few weeds which can survive the heat produced in commercial compost producing operations, so you can get it growing from non-peat composts.

If you ever discover the answer please tell us! Pull out all you see and don't let it seed. Eve so it still pops up everywhere doesn't it?