What makes you think its a grub that's causing it? There are many reasons why a lawn would be 'patchy'. Usual grub problems are caused either by leatherjackets (although its a bit late to have those now) or chafer grubs.

it could be from compaction or bitch urine if you have a dog

As Bamboo says, lawn problems are far more usually to do with cultivation problems - too much water, too little water, not enough light, poor nutrition, poor soil structure to name a few.

If you have found grubs in the lawn then you could have issues with Leatherjackets or chafer grubs - both are best controlled with nematodes that can be watered in and won't cause any issues with other wildlife or plants in the garden.

Cut and paste the following links for more info:

http://www.ecocharlie.co.uk/garden-care/pest-and-disease-control/nemasys-leatherjacket-killer.html

http://www.ecocharlie.co.uk/garden-care/pest-and-disease-control/nemasys-chafer-grub-killer.html