Follow the die-back along each branch and cut it out when you reach healthy wood. Conifers won't sprout new growth from brown wood but there looks to be enough healthy foliage to grow over and cover the resulting gaps. If the die-back continues after doing this, I'm afraid the tree is doomed and replacement is the only option.
Every year around this time my Tina Turner bush (it looks like her hair) does exactly that and every year I think it's time is up. I do nothing except wait for it to die.
It's still here!
Thanks Bob [assume not Flowerdew !] and Plant Pauper.
I will start snipping.The trees are only 4 years old [to me anyway].Weirdly, a big moth flew out when I moved a branch.Any possible connection ?
I did used to have a pony-tail many moons ago, but not I'm not Mr Flowerdew.
The moth will just have been hiding in there until the night came along.
The diseases which cause it are thought to be spread by aphids or other sap-suckers such as scale insects so keeping an eye out for those next year may help.