It may be a disease that affects conifers called phytoptera. A kind of mold that thrives on damp. It is said to have changed behaviour recently as regards treatment (forestry commission website), made it more difficult to treat.

Take a close look at the foliage that is still turning brown, you may have an aphid problem "cypress aphids" can cause rapid die back and browning especialy in drought conditions, you could also have a girdling root which would choke off the trees vascular system, if you think this tree is suffering from Phytophora??? (P. lateralis has been identified in Scotland) dig it up and burn it.
Looking at your photo your tree is a chamaecyparis ellwoodii and probably wont survive this infection, for now compleatly remove and burn if you can all the infected branches (they are well past recovery) if you find an aphid infestation spray the tree with a good insecticide, your local garden centre should be able to recomend a suitable chemical, fork around your trees base to improve airation and drainage checking for any obvious fungal activity.
Phytophora can be spread in ground water and by rain "splash" over watering can encourage it..

To echo Treeman89, I also think you may have an aphid problem causing the die off and brown patches. I was reading this (http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=132) today so serendipity! Read what the RHS says but he "Clues are left behind, including cast aphid skins and a black fungal growth (sooty mould)"

Certainly you need to cut out the brown areas ASAP.

Thank you very much for your replies. I have cut out the dead parts and inspected the rest very carefully but am unable to see anything in the way of an aphid infection or any other creature for that matter. I just can't work out what the problem is. I have 2 other conifers of the same kind close by and they seem fine.