It's chickeweed, pull it out by hand as soon as you see it and don't ever let it seed as you'll have it for ever more !!!

Looks like chickweed to me . Best thing is to pull it up or hoe. If it is chickweed it will soon be gone, it's an annual, but will seed ready for action replay next year.I have it every year and don't let it bother me. The birds love it.

I use it instead of cress/lettuce in sandwiches - it's nice in salads too and can be thrown into soups & stews, it's apparently very nutritious. I also read somewhere that having it in the garden decreases insect damage to other plants but I can't remember why or how :)