It's an oxalis - it grows from a tiny corm that stays in the ground when you try to weed it out. Oxalis corniculata is it's Latin name, also known as Creeping woodsorrel.

Thank you very much Beattie

it produces an explosive seedpod that will fling seeds well over a metre away, so if you dont want it weed it out before it sets seed.

We call it Sleeping Beauty and it's a pain - don't let it seed. It comes up everywhere and you'd wonder how it managed to spread itself so far even with exploding pods. But it is pretty!