I'm wondering who's informed you of that and what the symptoms are - seems a little early for leatherjacket larvae to be causing issues. Usually, the problem appears in spring, when the weather's a bit warmer, and birds peck at the grass, and the grass has brown patches - when you tug on the brown patches of grass, it pulls away from the top of the soil easily. A second way of checking is this - lay a sheet of black plastic or a tarpaulin on the lawn in the evening, leave overnight. Remove the plastic early next morning - if its leatherjackets, they should be visible on the top of the soil. The birds will take any sitting there, so you don't need to remove those ones.

There is a treatment you can use, but I don't think its warm enough yet - google Nemasys leatherjacket killer - you'll need to order online, but read the directions first, because these are nematodes which are mixed with water, then watered onto your lawn. If the air temperature isn't right, or you use it at the wrong time of year, it won't work.

After treatment some lawn maintenance would be valuable, in particular, aeration - leatherjackets tend to prefer poorly drained turf.

Get yourself a family of foxes.....they eat all the leatherjackets in my lawn. Admitedly the garden resembles the surface of the moon every morning, but i don't have leatherjackets ;-)

I had a problem with them last spring but the magpies kept pecking at the lawn I presume they were eating them.
Once I started cutting the lawn I had no more problems I wait to see if they return this year.