I would cut the diseased bits out. Cut down to good wood. Disinfect the secateurs with a wipe of methylated spirit between each cut to avoid spreading disease.
When you cut back the affected stems, if there is any discolouration in the wood where you cut it, keep taking it back further until the wood is white. That should remove any disease which started at the tip. If there is black or brown staining inside the wood all the way to the base, it's probably a fungal attack on the roots (like phytophthora) and probably best to remove the whole thing.Thanks for great advice.