Update
About two weeks after treatment it is begining to look like the nematodes are working. The brown patches are greening up after a quite a lot of a rain.
Original post
Earlier this year we suffered from leatherjacket damage, direct to the grass roots and subsquently when badgers dug up the grass to eat them. There is no longer any chemical control available for cranefies so we are tring nematodes. We got enough to treat 100m2 for about £20 from Amazon. 