I used to work in a large hotel here in Sussex which had Wellingtonias in the grounds. I noticed the same thing on them and can't remember where I read it or found out about it but was told this wasn't unusual, they did turn brown on some of the branches. It was a natural dieback. The trees or seeds came from Wakehurst Place originally so perhaps you could contact them for more information. They have been more than helpful to me in the past.

Many thanks Cammomile. I'll get in touch with Wakehurst Place & see what they can add.

How big are they out of interest?

Around 80-90ft. Just a sapling really in redwood terms