Leatherjackets are the larvae of the Crane Fly (or Daddy Long Legs). They develop beneath lawns, munching through the roots, until they pupate and eventually emerge as a harmless Crane Fly.
Parasitic nematodes, Steinernema feltiae, are available for killing Leatherjackets and this is the most effective method. They can be watered onto the affected area in autumn.
Alternatively, try covering the area with black polythene on warm nights in early summer after it has rained. The Leatherjackets often come to the surface and can be collected up and destroyed the next morning. You could also try encouraging wild bird species that will eat the grubs. Chickens can be very effective - although they can cause just as much damage as the Leatherjackets!