monkey puzzle tree

They can some times turn this colour due to pollution. They normally keep stems majority of the way down stem, but pollution can affect them causing them to drop. Or it may be natural as the plant has probably produced more growth up top, its naturally dropping a few lower down.

is there anything tat can be done to help or prevent this happening?

Not if it is the natural thing it does, and only pollution suggestion is move to a hill away from it all!
Have branched suddenly started to turn brown?

yes in the last 2 months but only the lower ones