Cut it back hard and it should romp away again.
Cotinus plants are tough and can be cut down to ground level each year and will make several feet of new growth in each season.

Try the scratch test on the part that seems dead. Scrape a small piece of bark back and see if the wood underneath is brown or green. If it's brown, that means the branch is dead, I'm afraid.

Once you've pruned it, give it a feed to give the poor thing a boost. Blood, fish and bone is a good fertiliser.

Thanks for those tips, it just seems strange that it stems from one trunk and then branches off to 3 separate branches, one branch is full of new growth, the other 2 seem dead. incidentally, the base of the main trunk has alot of new growth. i reckon it must be at least 25 years old, maybe time to cut off th dead branches, should i do this now or in the autumn?

No point in waiting much longer if you're CERTAIN that the branches are dead, especially if it looks ugly. You may have to shape it once the dead parts are off.