Do you own a dog[bitch in particular] as it does look like urine scorch.
rake at it a bit is there any obvious fungal growth there?
Have you sprayed weedkiller/pathclear and then walked on the grass before it has had chance to dry?

Could they be Chafer beetle larvae or Leatherjackets under the turf ???

Grubs chewing at the roots...as Badfish says.

I would agree with badfish, it looks to me like leatherjackets.

Same for me, it was only yesterday that I noticed the starlings going over my lawn like little hoovers feeling with their beaks for leather jackets.

Many thanks to all respondents. I think we are feeding the starlings too well so they are ignoring natural fodder. I guess we are in for some hard work.