Unfortunately once conifers go brown they don't grow back. I would be surprised if it is the weather as you say they are mature trees. There is a wee beastie that is targeting leylandii - damned if I can remember its name though!

Are they going brown all over, or is it just in patches, i know we have had along dry spell but i dont really think this would be the cause, i have a long hedge of leylandii over in france that tolerate very long dry spells and scorching temps, and, have not noticed any problems on my garden rounds, however, check for signs of the conifer aphid, are there signs of sooty mould aprox three inches inside the growth, its a big problem the conifer aphid, try and find the cause quickly because conifers will not respond once dieback occurs, could it be a fungus problem? any neighbour problems?, julien.

adult trees reach down to the water table generaly so as such dont need much watering . thats why the main root that goes strate down is called a tap root . this can change if the water table drops dramaticly because of very dry spells .