Posted: 05/08/2015 at 11:24
If it is as it sounds, Oxalis corniculata, then when you got it , you got it. World wide, it is now reckoned to be one of the most persistent and pernicious of weeds. Weed killer will deal with it, to some extent, but you need to keep at it and get every scrap of it, before it flowers.
Pulling it out just leaves lots of tiny bulbils behind which regrow. The seeds explode and spread widely.
I use a Flame gun weeder on the gravel paths. While it does not get rid of it completely, it has reduced the population.
PS. It also comes in the peat free composts as even the heat they generate does not kill the seeds. You also buy it when you buy plants as it is rife in Nurseries. too