Probably, yes.

once leylandii go brown they do not regrow from the old wood so even if its just a patch that area will not go green again

I agree with Spritzhenry.
Brown foliage is usually a sign of water deficiency. These trees are prolific growers and require plenty of water. Often requiring extra water during drought conditions. This is more likely when grown as a hedge. Alternatively it may be insect attack, though I doubt it myself.
To see if the tree is dead, scrape a piece of bark up with your fingernail below the brown foliage. If the innerbark is green then the tree is still alive and may recover in a few years. If its dry and brown all foliage above this (on this branch) is dead and should be removed.
I hope this helps.

I suspect the weather has got to the hedge - mind you I'm all for leylandii dying!