I think its lesser Celandine.

It has a pretty yellow flower and it disappears once the flowers go. If you dig around you will find little sausages which are the corms/bulbs. I have it in my garden and just leave it because it vanishes once the buttercup like yellow flowers are past. If you dig it up it has funny sausage like roots. I chuck them in the brown bin (recycling) if I unearth them when i'm gardening but without digging them out of the lawn I can't see any way of getting rid of them.

Lesser Celandine it is too. Ranunculus ficaria to give it its proper name and a right thug too. Almost impossible to eradicate, but at least it disappears in a few weeks time. Weedkiller does work, slowly in places where it is a nuisance, but like Scotsgran we tend to ignore it. I do have a white flowered one as well as R.f. Brazen Hussy which has bronze coloured leaves.

I like the celandine and the daisies in the lawn......I,ve got some primroses self seeded too.....it all looks so pretty

Only problem is the exploding seed pods so in a few years it gets absolutely everywhere.