You can buy a nematode treatment to be watered on called Steinernema (that's the name of the nematodes themselves, not the product) August to October is the time to use it, when the temperature is above a certain level. Google leatherjacket treatments and have a look. Otherwise, a sheet of plastic laid on the lawn overnight will bring them to the surface, remove the plastic in the morning and either pick them up and destroy, or let the birds have them - they love 'em!

...particularly Starlings!

I had just changed carpets and had some underlay which I used to cover the grass last night and came to check this morning but didn't see a sign of a single leatherjacket? Saying that it is summer and days are quite long so could it be because of that? Above all, is this problem related to leatherjacket or could it be something else?

You would see this kind of damage at this time of year were it leatherjackets, but there are myriad causes of die back of turf, from scalping through to fusarium patch, red thread, compaction, buried debris, drought, dollar spot disease. All of these have other symptoms, or only appear at certain times of year, which helps to identify cause. With leatherjackets, one of the signs is birds searching constantly on the lawn for grubs, and the patches caused are usually brown, and more usually evident in spring and early summer. You should be able to pull the brown grass away easily, and find legless grey grubs present in the soil too.