Leylandii go brown, mostly, through lack of water or wind damage, given the drought conditions the country's had for a few months and then the high winds recently it's going to have an effect.

thanks for that Louise .so its just a waiting game to see if it will recover :-)

There's an aphid that attacks Leylandii - cypress aphid.
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=492

Have a check and see if there are any on your plants.

I would agree with Beatie, most possibly the conifer aphid, check for signs of sooty mould aprox 3/4 inches in, they feed further down the shoots on the older wood, if it is conifer aphid then you need to act quick and spray with provado bugclear, and remember once conifer die back then it will not come back, julien.

Thank you all for your advice and i will have a look for the signs that you mentioned and try to deal with this aphid if its there and save the conifer if i can. again thank you all for your help :-)