I'm afraid that once some conifers start to go brown, they won't recover.

Yew does if you cut it back, but Leylandii and others don't regenerate.

Have a look and see if you can see any aphids on them - it just might be an insect attack - which you could spray.

Thanks again,i checked for insects and nothing apparent.
Very frustrating, it takes several years to arrive at size and shape of conifer you want but a few weeks to loose.
Cheers.

Yes - I'm sorry it's happened. :-(

Over the years i have come across this many times, the most obvious cause is down to the conifer aphid, you wont find them on the outer folige you have to look further in aprox trhree inches they are very difficult to see on the brown stems however a sure sign is sooty mould, another cause could be conifer scale, in both cases spray with provado bugclear and do this aprox four times a year, remember though once dieback occurs then leylandii will not re shoot, just one other possible cause have they been tightly clipped at the wrong time of the year, ie were they done at the backend last year before all the severe frosts?