Posted: 10/11/2014 at 13:35
That's dog lichen, it's not a weed. Treatment - none, but you need to improve the drainage drastically by getting a fork and going quite deep, 4-6 inches and lifting the lawn. Brush in good quantities of sharp sand and grit. You'll need to do this 5 or 6 times a year as your ground is compacted and the drainage is very poor. Shade is probably a problem where this is, sowing grass seed suitable for shady area's would help. You won't cure this easily unless you went to the expense of lifting the whole lot, then opening up the top 12 inches of soil and adding plenty of grit and sharp sand mixed with compost.