have a look for aphids, though usually once conifers have gone brown they don't green up again.

Sending sympathy. Don't want to depress you but I don't think feeding and watering will make any difference once it starts to do that. Our very similar conifer hedge is doing the same and has been gradually getting worse over the years. Its a question of which dies first, the hedge or me...
If you ever discover a cure please let us all know!

I would hold off on the watering. They don't like 'wet feet'. Give it time to dry out between waterings especially if you have clay. Once a week is sufficient plus whatever rain you get. Cut off the brown bits. They don't need fertilizer at all.

Thanks for the replies suppose I'd better start pricing timber for a fence !!

Well at least you won't have to keep clipping a fence and you can grow Clematis on a fence. But lots of other evergreens are good for hedging. A really dense hedge can be grown quite quickly with Lonicera nitida - its a rather boring dark green but there is a cheerful bright yellow one as well. (In fact there was an ID question about this a few days ago with a photo) But if privacy isn't an option there are so many great alternatives...I'd love to replace ours but its long, and gardeners algebra says a pence plus b pence + x pence