is this the first year you have noticed it? it looks like it is the beginning of a fairy ring. this is caused by a fungus in the soil. as the fungus grows out the ring forms and gets bigger over time. In the past I have just left it alone but my grass has been there as a place for the kids to play. if your lawn is a special feature of your garden then I can see why you wont want it.
this is the link to the RHS advice about fairy rings.

https://www.rhs.org.uk/advice/profile?PID=158

Many thanks. I will check in to the link you provided me. Not a play area for children now so I appreciate your advice on how to remedy it!

I'm thinking the same thing you are. It's patch work. Somebody filled in a bare spot with different kind of grass seed than the surrounding turf. It's 2 different types of grass. You can try to reseed or returf. It would be like playing match-game.

We have one of these too. Doesn't seem to have any mushrooms and the grass looks the same kind - the lawn was turfed before we bought the house but we have never reseeded any of it. We just treat it as one of life's little mysteries...