Funny you should say that here in derby have noticed quite a few goldcrest going brown, on the ones inspected, there appears to be no signs of pest ie the conifer aphid, they do like to be kept well watered and due to the very dry spring we are having, then this could be the problem, often though once browning occurs they seldom survive, i would give it some copious amounts of water and see what happens, if it perks up then gently shake the dead parts away. julien.