It's a form of oxalis - you can dig them out, but you need to dig deep, shake off the soil carefully, and keep each one intact. There's often a cluster of small, brownish pod like things at the base of the stems, then roots, and often a whitish bulbous object. All these should be removed. Take the topgrowth and roots, and leave the whitish bit, and they'll grow again.

Sad to say this is the one plant which has to be dealt with by weedkiller. And even then it will keep on coming back. Don't ever ever ever let it flower or you will get seedlings appearing for the next 7 years!
By the way it is Oxalis cornuta, also called Bermudan buttercup which is odd as it does not come from Bermuda and it is not a buttercup.

Hi, Thanks very much for your help, looks like I've got a lot of digging ahead.

Exploding seedpods! The seeds are projected in all directions. Pretty, but I've not been able to get rid of it by digging up.

After digging I was going to put garden fabric down with lots of Bark on top. This is in the front garden border so its not a massive area.

I have managed to clear a large area, but even now, after 5 years, bits of it reappear. I just keep a close eye and dig it out as soon as it appears.

Same here. I even get odd bits popping up in the pots in the conservatory!

It's currently in 2 planted pots on my balcony - waiting till I repot to get it out.

It comes ready packed in the compost.

Crete is overrun with it

i think its quit atractive and theres a more yellow veriaty i believe .

It is quite attractive - but if you leave it, pretty soon that's all you'll have everywhere...

ow i see i thaught bigger shrubs etc would out compete it but well worth knowing . my mum has it between her paving slabs and it stays there and looks quit attractive .