Any sign of grubs in the soil?

Have you noticed birds pecking at the lawn at all? When you lift up the clumps of dying grass, have you seen any larvae beneath? Only it sounds like chafer grub (especially if you have sandy soil) or leatherjacket infestation. You can check if it's leatherjackets by watering a patch of lawn, laying a sheet of plastic over it at dusk, then remove it in the morning - if its leatherjacket larvae, they'll be on the surface of the grass.

That is what I was wondering too. The magpies are digging holes all over my grass this year.

thanks all - for these comments - I think I may have leatherjackets - and will try the experiment you advise - Bamboo.