Posted: 23/08/2015 at 21:03
The infamous and ubiquitous Oxalis cornuta is what this sounds like. When you got it, you got it. It even comes in the non-peat compost which one is encouraged to use these days.
Sorry, but weed killer is the only way to even remotely control it. I use a Flame gun on the gravel paths, but even then it returns.