This is a lichen, Ddd. It indicates that your lawn is damp and poorly drained - indeed, I can also see moss in the pictures. This is very common and afffects most of the lawns in our village.

Its specifically called Dog Lichen (Peltigera canina) and as said above indicates poor drainage especially in conjunction with the moss.

Scarifying the lawn, spiking and feeding should all help in time to reduce the incidence of it in future.

Moss killer will get rid of this, but unless you improve the health of the lawn by doing as Fractal suggests, it will return.

I have the same on my grass and its spreading, I was told that bowling clubs treat there grass with something to get rid of it but I don't no what and what the name of this thing on the grass is to ask for help. Scarifying it has made it worse as the sporse get moved about, digging it out is a hard job on our grass. Moss killer didn't work on mine and we got rid of the moss fed the grass and was left with bald spots when we pulled it out!