It is probably the lesser celandine. Dig out as many of them as you can, including the small brown tubers. Or you could spray them with a glycophosphate type of weedkiller. They are a nuisance though.

Mine has too :-((
I wonder why i've got it now, i've not had it here before ?

Thank you very much. I was thinking of trying glycophosphate but was worried about the plants around the celandine. The problemis so bad now so I will have to use a chemical. Thanks again.

Aileenthegardener

you can paint it on with and old pstry brush, may get odd looks from the neighbours ! or lift the plants you want then spray. replanting a day later. glycophosphate becomes inert on contact with the soil, so you can replant. It would be worth washing the soil off the roots to remove any celandine tubers.

er, i thinkin htat marsh marigold is Caltha palustris, not celandine - celandine is Ranunculus ficaria, or comon name PILEWORT. in russia it is call chistotel, weer it use a lot as herb.
I copy/paste this:
J. R. R. Tolkien mentions this plant (and many others!) when he describes spring in Ithilien: "Great ilexes of huge girth stood dark and solemn in wide glades with here and there among them hoary ash-trees, and giant oaks just putting out their brown-green buds. About them lay long launds of green grass dappled with celandine and anemones, white and blue, now folded for sleep; and there were acres populous with the leaves of woodland hyacinths: already their sleek bell-stems were thrusting through the mould." The Two Towers, Book IV, Ch 7, 'Journey to the Cross-roads'
hopin htis all helpin you :)