Looks like lesser celandine to me which can be rather invasive, if you want to keep you'll need to remove some of it regularly...

the leaves don't look glossy enough for celandine. I have seen a similar looking plant that was one of the Doroconium species.
could you take a closeup of the leaves and the flowers and add them to your question.
Have you dug any up? Do they have a little brown corm/nodule at the base or a rhizome or just normal roots.

welcome to GoY too :o)

I agree with Moon grower, I think they are lesser celandine. It depends on where they are growing on how big they get as well. I have some which my Sister gave me in various spots (as I like them) and they vary from almost flat in dry bad soil to looking as above. My Sister has some in her soil, which they obviously love, which are HUGE and look more like Marsh Marigolds. She can't get rid of hers, even with vigorous digging - which isn't going to happen here!

I absolutely love celandine. It's one of the earliest spring flowers and really cheers up hedgerows and woodland paths etc, but in the garden border it is a menace which multiplies as you watch! This year seems to be a particularly good one for celandines and I am struggling in this dry weather to dig them out of the border without leaving the little root nodules behind.