If you're worried about the toadstools, twist them off and dispose of them. These are just the fruiting bodies of underground mycelium, which are like a root system, and they're "in flower" now. You may get more at the same time next year.

I agree, they are extremely difficult to get rid of without killing the grass too and should be just brushed away with a stiff brush if there are lots of them. As said, they will probably appear again next year at around the same time.

There are a large number of species of fungi that live in or can potentially live in turf for one reason or another. Most live on dead thatch in the lawn which you appear to have from looking at this photo. If they are of this type, scarifying the lawn in spring and feeding will help.

Another group that appear in lawns usually occur in new lawns that have been laid over wood or have bits of wood left in the soil that these saprophytic fungi live on. These will disappear after a couple of seasons when the wood has rotted away.

The third and last group are associated with the roots of trees and live in symbiosis with the trees root system. These are completely harmless and indeed vital for the health of the tree.

Incidentally, the majority of fungi are pretty harmless with the worst two (in the British Isles) being pure white. The delightfully named Destroying Angel and Death's Cap are those two so no worries there. The worst you could expect from most is a slightly upset stomach or seeing double if alcohol is taken at the same time......or a shortened period of time between trips to the toilet!

Fractal. Do you know of anything that will kill mushrooms (I accept it will kill the grass as well) but would allow me to reseed the killed off area with grass or lay new turf almost immediately. I have a customer driving me mad because she has mushrooms in her lawn .

I only know of Armillatox.

Go here, http://www.armillatox.com/

then click on fairy rings.

Thanks for that. Have you any experience of using it?

Yes, but now on a lawn. It worked though.