The Yellow Fumitory (Corydalis lutea) is now naturalized all over London. You will normally find them in church yards along cool and damp walls where they put up quite a show.
If conditions are ideal, it will eventually come to your garden. I was delighted when it appeared in mine on a pile of old bricks.
For the impatient gardeners amongst us, young plants transplant quite easily if you get them with the roots intact.
Corydalis lutea are not normally stocked in garden centres as many gardeners treat this pretty plant as a weed, but not me, I make sure to keep one or two seedlings and then I will pull out the older bunches which always leave loads of seedlings to choose from.
They flower non-stop the whole summer and require zero attention!