Looks like algae to me. It grows on wet thin shaded grass. This is the RHS page on the problem

http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=414

I love the exact match between your photo and the one on the above page!

its either a lichen or a fungus. I cant remember which. It grows when the soil is compacted. if you fork the soil to improve the drainage nd rake this off the lawn will recover.

we have done some raking and some forking but maybe not enough ~ definitely right about the shaded grass.
thank you both.

just looked up the RHS site ~ many thanks beattie, looks just the same ~ dog lichen i think it meant ~ i shall be out with the fork tomorrow after work!

You're welcome Stickitoffee :-)

:o)

Yes, Dog Lichen....tell me about it. My front lawn in part shade with bad drainage at the bottom lower end gets it every year!

not nice is it!! i like moss, it fascinates me and its green but this stuff is not good!

Being a lichen, it's a composite organism, part plant (algae), part fungus. Interesting but not in my lawn. Not its fault of course, the conditions at one end though suit it.

Could you replace the naff grass in that area with bark/gravel etc? We replaced poor grass at the bottom of the garden with gravel and it's much better esp in winter.

that is a thought volunteer but there wouldnt be much lawn left ~ its nearly all moss.
i shall try the forking to get air in first, its funny cos we have very free draining soil

Perhaps you've had a lot of rainfall? The water could be draining away, but being replaced? That's what happens here.

that could be, i dont think we had any of that dog lichen in the back lawn last year, and i think that was a better summer?

Although you say that your soil is free-draining, I expct thatthe part under the lawn is very compacted, so a good aerate will do the world of good! (for you, as well!)

ok, thank you. was going to do it today but its cold dark and wet ~ maybe tomorrow?

A treatment with moss killer will see that off quickly, Stickitoffee, but it is, as the others suggest, a problem in poorly maintained lawns. Aerate as suggested when you get the chance, and make sure you use a good lawn food at least twice during the season next year, starting from end of April.

do i put the moss killer on now bamboo?

You can try it now if you want, but I'd be inclined to wait till very early Spring, or early April, when you can use a combined lawn weed/feed/mosskiller - if there's actual moss present as well, it'll go black, and that's not nice to look at all winter. If you wait till Spring you can then rake out the dead stuff and reseed where necessary. If you just want to treat that patch with a simple mosskiller though, then give it a go.

thanks again bamboo