Ranunculus ficaria, lesser celandine.

Nasty weed. Start getting rid of it. then realize that doing so will require considerable time as well as continuing effort.

Latest name is Ficaria verna, I believe. It is a British native wild flower and in the great outdoors is a delight. (the OP is in the UK) If you have it in your garden however, you will have it for ever and at this time of year it will smother delicate things. It will disappear in the summer to reappear next spring. in the US it is an introduced invasive alien.